Power plant with energy recovery from fuel storage

ABSTRACT

Power plant systems and processes are described that enable recovery of at least a portion of the fuel storage energy associated with a storage system for supplying fuel to the power plant systems. A first embodiment of an energy-recovery power plant system includes at least one fuel storage container and at least one expander that can receive fuel from the fuel storage container at a first pressure and provide the fuel to the power plant at a second pressure that is lower than the first pressure. A second embodiment of an energy-recovery power plant system includes a first conduit fluidly coupling the fuel storage container and the power plant for delivering fuel from the fuel storage container to the power plant and at least one regenerative thermodynamic cycle engine thermally coupled to the first conduit such that heat may be exchanged between the fuel and a working fluid for the regenerative thermodynamic cycle engine.

CROSS REFERENCE TO RELATED APPLICATION

[0001] The present application claims priority from prior pending U.S.provisional application No. 60/260,608, filed on Jan. 9, 2001, which isincorporated herein by reference.

FIELD

[0002] The present disclosure relates to a power plant system thatincludes a fuel storage system.

BACKGROUND

[0003] Fuel cells provide an environmentally friendly source ofelectrical power. One form of fuel cell used for generating electricalpower, particularly for vehicle propulsion and for smaller scalestationary power generation, includes an anode channel for receiving aflow of hydrogen gas, a cathode channel for receiving a flow of oxygengas, and a polymer electrolyte membrane (PEM) which separates the anodechannel from the cathode channel. Oxygen gas entering the cathode reactswith hydrogen ions that cross the electrolyte to generate a flow ofelectrons. Environmentally safe water vapor is also produced as abyproduct.

[0004] Despite the great attractiveness of fueling future mobile andstationary power plants with hydrogen, severe technical and economicbarriers are presented by the supply and storage of hydrogen (as eithera compressed gas or a cryogenic liquid). A substantial amount ofmechanical energy must be invested to compress and/or refrigeratehydrogen fuel being stored in any of these hydrogen storage systems. Inthe absence of any system for recovering energy as hydrogen fuel isreleased from storage, this energy loss seriously impairs the overallefficiency and economic viability of the so-called “hydrogen economy”,as popularly represented by distributed fuel cell energy systems usinghydrogen generated by renewable energy sources such as solarphotovoltaic power. In particular, the economic viability of hydrogenand natural gas as fuels particularly for vehicular propulsion has beengravely compromised by the loss of energy required to compress orliquefy these fuels in the fuel supply infrastructure. The prospects forwide application of hydrogen energy systems based on fuel cells could begreatly enhanced by development of a system for recovering “hydrogenstorage energy” to improve overall energy efficiency.

[0005] Specifically, hydrogen may be stored at substantially ambienttemperature as a compressed gas in high-pressure vessels, or in solidsolution within a metal hydride canister. Hydrogen may alternatively bestored at low temperatures (e.g., about 77 K to about 200 K) as acompressed gas in contact with an adsorbent (e.g. active carbon), or atmuch lower temperature (˜20 K) as cryogenic liquid. Some researchers arecurrently investigating hydrogen storage at substantially ambienttemperature as a compressed gas in contact with an advanced adsorbent(e.g. nanofiber or nanotube carbon).

[0006] Each of the above physical techniques for hydrogen storagerequires a substantial investment of “hydrogen storage energy”(typically as compression energy) to achieve the required workingstorage pressure and to provide any required cryogenic refrigeration.There has been a lack of practical devices and methods for recoveringhydrogen storage energy to enhance the performance and efficiency of thefuel cell power plant, particularly for small-scale fuel cell powerplants. There are some examples of hydrogen storage energy beingrecovered for ancillary uses. One example involves recovering a portionof hydrogen storage energy for air conditioning for passengercompartment comfort, where hydrogen fuel being released from cryogenicstorage may be used as a refrigerant. Another example involves using theendothermic heat of hydrogen release from a metal hydride as a heatsink.

[0007] One way to improve the performance of a PEM fuel cell system isoxygen enrichment of the air supplied to the cathode. Boosting theoxygen partial pressure over the fuel cell cathode will enhance fuelcell stack voltage efficiency at a given current density. Alternatively,oxygen enrichment can enable fuel cell operation at higher currentdensity with reduced voltage drop, thus reducing the size and capitalcost of the equipment.

[0008] Pressure swing adsorption (PSA) systems can provide a continuoussupply of enriched oxygen while also removing any contaminant gas orvapor components that may be detrimental to the fuel cell. PSA systems(including vacuum pressure swing adsorption systems (VPSA)) separate gasfractions from a gas mixture by coordinating pressure cycling and flowreversals over an adsorber or adsorbent bed that preferentially adsorbsa more readily adsorbed gas component relative to a less readilyadsorbed gas component of the mixture. The total pressure of the gasmixture in the adsorber is elevated while the gas mixture is flowingthrough the adsorber from a first end to a second end thereof, and isreduced while the gas mixture is flowing through the adsorbent from thesecond end back to the first end. As the PSA cycle is repeated the lessreadily adsorbed component is concentrated adjacent to the second end ofthe adsorber, while the more readily adsorbed component is concentratedadjacent to the first end of the adsorber. As a result, a “light”product (a gas fraction depleted in the more readily adsorbed componentand enriched in the less readily adsorbed fraction, here oxygen andargon) is delivered from the second end of the adsorber, and a “heavy”product (a gas fraction enriched in the strongly adsorbed components,here nitrogen, water vapor, carbon dioxide, and any contaminants) isexhausted from the first end of the adsorber.

[0009] The conventional system for implementing pressure swingadsorption or vacuum pressure swing adsorption uses two or morestationary adsorbers in parallel, with directional valving at each endof each adsorber to connect the adsorbers in alternating sequence topressure sources and sinks. This system is often cumbersome andexpensive to implement due to the large size of the adsorbers and thecomplexity of the valving required. Further, the conventional PSA systemmakes inefficient use of applied energy because of irreversible gasexpansion steps as adsorbers are cyclically pressurized anddepressurized within the PSA process. Conventional PSA systems could notbe applied to fuel cell power plants for vehicles, as such PSA systemsare far too bulky and heavy because of their low cycle frequency andconsequently large adsorbent inventory.

[0010] A serious challenge for oxygen enrichment by PSA or VPSA usingnitrogen-selective zeolite adsorbents arises from the stronglyhydrophilic nature of those adsorbents. Water adsorption fromatmospheric humidity will deactivate the adsorbent. For continuouslyoperating industrial PSA plants, this problem is solved by using thefeed end of the adsorbent bed (typically loaded with alumina desiccant)to dry the feed air. In intermittent operation, adsorbed water in thedesiccant layer may diffuse into the nitrogen-selective adsorbent zoneand cause deactivation of that adsorbent during shutdown intervals.Hence, it is very desirable that as much water as possible be removedfrom the feed air before that air enters the PSA unit, in order toreduce the humidity challenge to satisfactory sustained operation of thePSA under intermittent operating conditions.

SUMMARY

[0011] Various power plant systems and processes are described hereinthat enable recovery of at least a portion of the fuel storage energyassociated with a storage system for supplying fuel to the power plantsystems. In particular, a first embodiment of an energy-recovery powerplant system includes at least one fuel storage container and at leastone expander that can receive fuel from the fuel storage container at afirst pressure and provide the fuel to the power plant at a secondpressure that is lower than the first pressure. A second embodiment ofan energy-recovery power plant system includes a first conduit fluidlycoupling the fuel storage container and the power plant for deliveringfuel from the fuel storage container to the power plant and at least oneregenerative thermodynamic cycle engine thermally coupled to the firstconduit such that heat may be exchanged between the fuel and a workingfluid for the regenerative thermodynamic cycle engine. An expander and aregenerative thermodynamic cycle engine may be combined for energyrecovery from cryogenic fuel storage in a single power plant system.

[0012] In such power plant systems, mechanical power and/or thermalenergy may be recovered from the fuel storage systems. For example, aprocess is disclosed herein that involves providing a compressed fuelgas or a cryogenic liquid fuel, releasing the fuel from a fuel storagesystem, and generating mechanical power and/or a refrigeration effectfrom the releasing of the fuel.

[0013] The disclosed power plant systems and processes are particularlyuseful for electrical current generating systems that include a fuelcell. Certain versions of such systems can include at least one oxidantgas delivery system that can produce oxidant-enriched gas for deliveryto the fuel cell. The oxidant gas delivery system may be a pressureswing adsorption system that includes at least one device that iscoupled to (and at least in part powered by) the expander and/orregenerative thermodynamic cycle engine.

[0014] Also described are processes for providing hydrogen to fuel cellsin such electrical current generating systems. One disclosed processscheme involves releasing hydrogen from a hydrogen fuel storage systemto provide a compressed hydrogen gas stream, introducing the compressedhydrogen gas stream into at least one expander resulting in alower-pressure hydrogen gas stream, and introducing the lower-pressurehydrogen gas stream into a fuel cell. Another disclosed process schemeincludes providing a regenerative thermodynamic cycle engine having aworking fluid. Heat is transferred from the regenerative thermodynamiccycle engine working fluid to the hydrogen stream and heat istransferred from an air feed stream to the regenerative thermodynamiccycle engine working fluid. The air feed stream may be introduced intothe fuel cell.

[0015] The foregoing features will become more apparent from thefollowing detailed description of several embodiments that proceeds withreference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 shows an axial section of a rotary PSA module.

[0017]FIGS. 2 through 5B show transverse sections of the module of FIG.1.

[0018]FIGS. 6A and 6B are simplified schematics of PEM fuel cell powerplants with a PSA unit for oxygen enrichment, with feed airrefrigeration and optional mechanical energy recovery from the hydrogenstorage system.

[0019]FIG. 7 is a T-S diagram for hydrogen, showing direct expansion andtwo stage Stirling cycle energy recovery options.

[0020] FIGS. 8 to 10 are simplified schematics of PEM fuel cell powerplants with a PSA or VPSA unit for oxygen enrichment.

[0021]FIGS. 11A and 11B show a combined direct expansion and Stirlingcycle system for energy recovery.

[0022]FIG. 12 is a T-S diagram for hydrogen, showing a three stageStirling cycle energy recovery option.

[0023]FIG. 13 shows a fuel cell power plant with energy recovery fromhydrogen storage, and with provision for defrosting the heat exchangecoil in the condenser of the feed air chilling system so as to preventice build-up.

[0024]FIGS. 14 and 15 are schematic cross-sections of a rotary valvepressure swing adsorption apparatus.

[0025]FIG. 16 shows a fuel cell power plant with a Brayton cycle systemfor energy recovery.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS

[0026] Representative embodiments are described below with reference tothe drawings. The following definitions are provided solely to aid thereader, and should not be construed to have a scope less than thatunderstood by a person of ordinary skill in the art or as limiting thescope of the appended claims.

[0027] “Ambient pressure” denotes about 1.0 bar absolute with allowancefor changes of elevation and barometric conditions.

[0028] “Ambient temperature” denotes about −40 to about 35° C.

[0029] “Light gas or light product” denotes gas enriched in a lessreadily adsorbed component.

[0030] “Rotary bed pressure swing adsorption apparatus” is an adsorptionapparatus where adsorbers with adsorbent material are rotated relativeto stationary valves for introducing and withdrawing gas streams.

[0031] “Rotary valve pressure swing adsorption apparatus” is anapparatus where valves for introducing and withdrawing gas streams arerotated relative to stationary adsorbers housing adsorbent material.

[0032] “Rotary pressure swing adsorption” includes, but is not limitedto, a rotary bed pressure swing adsorption apparatus, a rotary valvepressure swing adsorption apparatus, or a pressure swing adsorptionapparatus that includes rotating adsorbers and rotating valves.

[0033] The disclosed systems and processes may be applied more generallyto recovery of energy (both as mechanical work and as refrigeration)from compressed gas or cryogenic liquids for fuel cell power plants andcombustion engine power plants. The recovered energy typically is atleast a portion of the energy initially required to compress or liquefythese fuels in the fuel supply infrastructure. Recovery of such “fuelstorage energy” involves recovering at least a portion of the energyavailable when the fuel is released from the fuel storage system to thefuel delivery system.

[0034] The fuel storage system may be any system that stores a volume offuel for delivery to a fuel cell or a combustion engine. According toparticular embodiments, the stored fuel is any type of substance thatexists as a gas at ambient temperature and pressure conditions, but thatis stored in the fuel storage system as a compressed gas, a compressedliquid, a cryogenic liquid, a compressed cryogenic liquid, a sorbate ona physical adsorbent at ambient or cryogenic temperature, or a metalhydride compound. Examples of such fuels include hydrogen, natural gas,methane, and propane. The compressed gas (e.g., hydrogen or methane)typically is stored at ambient temperature at a pressure of greater thanabout 100 bars and up to about 700 bars absolute in the case ofhydrogen.

[0035] The fuel storage system can include any devices or mechanismscapable of providing the fuel to the fuel delivery system at a pressuregreater than ambient pressure. For example, the fuel storage system mayinclude at least one container for the fuel such as a pressure vesselfor compressed gas, a dewar for liquefied gas, a canister for a gassorbent, or a container for a hydrogen gas-producing substance.Alternatively, the fuel storage system could include a pipeline as apressure vessel container in instances of remote or distributed powerplant systems. The fuel storage system also includes a mechanism forreleasing the fuel to the fuel delivery system. Such fuel releasemechanism may include valves, flow controllers and similar devices thatcan provide controlled release of a compressed gas or cryogenic liquid.Fuel also could be released by a mechanical device (e.g., a piston)coupled to the expander. Additional possible fuel release mechanismsinclude devices that trigger the fuel release by conditioning the fuelsuch as, for example, a heat exchanger to raise the temperature of thefuel for desorbing the fuel from a sorbent under pressure. The fuelstorage system fluidly communicates with the fuel delivery system viathe fuel release mechanism system and, typically, at least one conduit.

[0036] The fuel delivery system may include any type of device useful totransport or propel the fuel through the overall fuel cell system orcombustion engine system and/or any type of device useful to enhance thefuel quality. Illustrative fuel transport devices include compressorsand pumps for fuel to be pressurized, and expanders for fuel gas to bedepressurized. Illustrative fuel enhancement devices include fuelpurification devices such as adsorption modules, filters, gas separationmembranes, desiccant traps, and pumps or mixers for mixing additionalcomponents with the fuel.

[0037] The energy recovered from release of the fuel from the fuelstorage system can be converted into energy that is useful to the fueldelivery system, or to deliver energy supplemental to the power plant.For example, the recovered energy may be converted into mechanical powerfor driving at least one fuel transport device. Another examplecontemplates converting the recovered energy into refrigeration forimproving the performance of at least one fuel enhancement device.

[0038] According to one embodiment, an energy-recovery expander isutilized to convert the fuel storage energy into mechanical power. Inparticular, the expander receives fuel from the fuel storage systemwherein the fuel is at a first higher pressure. The first pressure maybe the storage pressure of compressed fuel gas, or a desorption pressureachieved by warming an adsorbent or metal hydride containing the fuelgas, or a pressure attained by pumping the fuel from storage as acryogenic liquid before vaporization. The fuel is typically in a gaseousstate as it enters the expander. The expander reduces the gas pressureby allowing the gas to expand approximately isentropically. Thus, thefuel gas exits the expander at a second lower pressure. The expanderextracts work from the pressurized fuel gas while expanding the gas.According to certain presently disclosed embodiments, the work obtainedfrom the expander is converted to mechanical power by coupling theexpander to a mechanical load such as a compressor, a vacuum pump, aliquid pump (e.g., a coolant pump for the fuel cell), electricgenerator, an adsorber rotor, or a propulsion device such as wheels of aroad or rail vehicle incorporating the disclosed power plant systems.The coupling may be, for example, a mechanical coupling, an electricalcoupling, or a combination of mechanical and electrical coupling.

[0039] Any type of expander may be used in the disclosed systems andprocesses. Illustrative expanders include positive displacementexpanders and impulse turbines. Positive displacement expanderstypically include at least one chamber that receives the gas wherein oneor more walls or surfaces of the chamber can move under the gaspressure, thus expanding the volume of the chamber. The positivedisplacement expanders may be provided with sequentially increasingdisplacement toward the lowest pressure stage or as a rotary positivedisplacement expander for each stage. Examples of such expanders includea multistage piston expansion machine wherein the first stage cylinderis smaller than the cylinders of subsequent expansion stages, with thedisplacement of each stage approximately proportional to the intakepressure of the gas to that stage. Alternative rotary positivedisplacement devices (also known as rotary engines) include scrollmachines, twin screw machines, and other machines with twincontra-rotating and intermeshing rotors.

[0040] The impulse turbines may be provided with stages having partialadmission nozzles, and operating at very high speed, for example 100,000RPM in the case of low molecular weight of hydrogen. For example, athree stage impulse turbine may have three separate wheels, each with apartial admission nozzle. Alternatively, the impulse turbine for severalstages may have one wheel using re-entry nozzles and return ducts todefine the separate stages of expansion in the wheel over correspondingangular sectors of the casing, as described for two or three stages byO. E. Balje in section 5.2.3 of “Turbomachines” (Wiley-Interscience,1981).

[0041] The disclosed systems and process are particularly useful inpower plants that include a PSA (e.g., VPSA) oxidant enrichment system,especially an oxygen enrichment system. The recovered mechanical workassists powering of any part or device of the PSA system such as a feedair compressor, vacuum pump, rotary adsorbent bed and rotary adsorbervalve. In addition, refrigeration obtained by release and expansion ofthe stored hydrogen or natural gas is applied to cooling and drying offeed air to the PSA (e.g., VPSA) unit. There are numerous applicationswhere oxygen enrichment may improve the performance of a fuel cell orengine power plant. Hence, the efficiency of fuel cell or engine powerplants fueled by hydrogen or natural gas can be significantly improvedby the recovery of mechanical expansion energy and refrigeration fromthe hydrogen or natural gas storage system.

[0042] According to certain embodiments, the disclosed methods andsystems enable recovery of a portion of the fuel storage energy to beexpanded as mechanical energy to compress air into the fuel cell airhandling sub-system, which is preferably provided as a pressure swingadsorption (PSA) oxygen concentrator. Mechanical power that is generatedas hydrogen is released from storage may be applied, for example, todrive any mechanical-powered or -driven apparatus such as a compressor,blower pump, vacuum pump, or propulsion means of a vehicle incorporatingthe power plant. According to one example, the generated mechanicalpower can drive or assist driving a feed air blower for the oxygen PSA,as well as optionally a vacuum pump for reducing the adsorberregeneration pressure of the PSA which is then operating as “vacuumpressure swing adsorption” (VPSA). Oxygen generation by VPSA is moreenergy efficient than by PSA, as fractional yield of oxygen from air isenhanced by vacuum.

[0043] Methods and systems for cooling of feed air to the PSA or VPSAunit are also described. The cooling effect is provided by expansion ofstored hydrogen and, in certain embodiments, as refrigeration provideddirectly from release of cold hydrogen from a cryogenic hydrogen storagesystem. Refrigeration may also be provided by the endothermic uptake ofheat by a hydrogen storage adsorbent bed or metal hydride bed. Coolingthe feed air will reduce its saturation humidity in relation to thedegree of cooling, reduce the volume flow to be compressed resulting inreduced compression power load, and reduce the need for radiator coolingto reject waste heat from fuel cell power plants on vehicles. Moreover,this cooling of the feed air further reduces or eliminates the need foraftercooling of the compressed air before being admitted to the PSAunit. In addition, the cooling could be employed to cool a combustionengine or fuel cell coolant.

[0044] Direct expansion and regenerative thermodynamic cycle embodimentsare disclosed for recovery of mechanical work from release of hydrogenfrom cryogenic liquid storage. Illustrative regenerative thermodynamiccycles include regenerative Brayton cycles and Stirling cycles. Theseembodiments may be used in a refrigeration mode to augment the coolingof feed air to the PSA unit and the fuel cell. Direct expansion andregenerative thermodynamic cycle systems may also be used to recoverwaste heat from the fuel cell stack, from exhaust streams from the fuelcell, or combustion of the anode exhaust gas. Such fuel cell energyrecovery systems can augment the amount of mechanical energy obtainedfrom recovery of hydrogen storage energy, condense and recover waterfrom the fuel cell exhaust, and reduce the fuel cell cooling load forvehicle applications.

[0045] According to a further embodiment that does not include a PSAmodule, the expander is fluidly coupled to the fuel cell to deliver thefuel directly from the expander to the fuel cell. In the case ofhydrogen fuel and a polymer electrolyte membrane fuel cell, the expanderis fluidly coupled to the fuel cell anode. The expander may bemechanically coupled to (and thus power) a compressor that delivers anair feed gas directly to the fuel cell cathode without anyoxygen-enrichment by a PSA module.

[0046] A particularly useful electrical current generating system mayinclude a PEM fuel cell, an oxygen gas delivery system, and a hydrogengas delivery system. The PEM fuel cell includes an anode channel havingan anode gas inlet for receiving a supply of hydrogen gas, a cathodechannel having a cathode gas inlet and a cathode gas outlet, and anelectrolyte in communication with the anode and cathode channel forfacilitating ion transport between the anode and cathode channel. Theoxygen gas delivery system is coupled to the cathode gas inlet anddelivers air or oxygen (e.g. oxygen enriched air) to the cathodechannel.

[0047] The oxygen gas delivery system may simply be an air blower.However, for superior performance it incorporates an oxygen pressureswing adsorption system, preferably including a rotary pressure swingadsorption module for enriching oxygen gas from air. The rotor includesa number of adsorbers with flow paths between respectively first andsecond ends of the adsorbers and with adsorbent material therein forpreferentially adsorbing a first gas component in response to increasingpressure in the flow paths relative to a second gas component. Thepressure swing adsorption system also may include compression machinerycoupled to the rotary module for facilitating gas flow through the flowpaths for separating the first gas component from the second gascomponent. The stator includes a first stator valve surface, a secondstator valve surface, and a plurality of function compartments openinginto the stator valve surfaces. The function compartments include a gasfeed compartment, a light reflux exit compartment and a light refluxreturn compartment.

[0048] In one variation, the compression machinery comprises acompressor for delivering pressurized air to the gas feed compartment,and a light reflux expander coupled between the light reflux exitcompartment and the light reflux return compartment. The gasrecirculating means comprises a compressor coupled to the light refluxexpander for supplying oxygen gas, exhausted from the cathode gasoutlet, under pressure to the cathode gas inlet. As a result, energyrecovered from the pressure swing adsorption system can be applied toboost the pressure of oxygen gas delivered to the cathode gas inlet.

[0049] The oxygen gas delivery system is coupled to the cathode gasinlet and delivers oxygen gas to the cathode channel. The hydrogen gasdelivery system supplies purified hydrogen gas to the anode gas inlet,and may have provision for recirculating hydrogen gas from the anode gasexit back to the anode gas inlet so as to maintain adequate humidity atthe anode gas inlet. The hydrogen gas delivery system is fluidly coupledwith the hydrogen fuel storage system.

[0050] In one embodiment, the oxygen gas separation system comprises anoxygen pressure swing adsorption system, including a rotary modulehaving a stator and a rotor rotatable relative to the stator. The rotorincludes a number of adsorbers for preferentially adsorbing a first gascomponent in response to increasing pressure in the flow paths relativeto a second gas component. The function compartments include a gas feedcompartment and a heavy product compartment. Alternatively, theadsorbers may be stationary, with cooperating first and second rotarydistributor valves respectively for admitting feed air and releasingnitrogen enriched exhaust from the first ends of the adsorbers, and fordelivering oxygen enriched gas from the second ends of the adsorbers.

[0051] In one variation, the oxygen pressure swing adsorption systemincludes a compressor coupled to the gas feed compartment for deliveringpressurized air to the gas feed compartment, and a vacuum pump coupledto the compressor for extracting nitrogen product gas from the heavyproduct compartment. Most PEM fuel cell systems operate at ambient toabout 3 bars pressure. As feed pressure and the overall working pressureratio of the PSA are reduced, productivity and recovery of a simplecycle deteriorate. At very low feed pressures (e.g. 2-3 bars), the PSAmay need supplemental vacuum pumping to widen the working pressure ratioand achieve high recovery. In typical conventional PEM fuel cell fuelcell systems, all mechanical power for air handling compression and anyoxygen PSA units must be provided as electrical power by theappropriately sized fuel cell stack. In the presently disclosed systemsand processes, the recovered mechanical energy can be employed to atleast partially power the compressor and/or vacuum pump.

[0052] As mentioned above, the disclosed processes and systems also areapplicable to combustion engine systems, particularly combustion enginesystems that include a PSA module and utilize hydrogen and/or oxygen ascombustion fuel. Illustrative combustion engine systems are described incommonly assigned, co-pending U.S. patent application entitled “FeedComposition Modification for Internal Combustion Engines” filed Oct. 26,2001, the disclosure of which is incorporated herein by reference.

[0053] FIGS. 1-5

[0054]FIG. 1 shows a rotary PSA module 1, which includes a number “N” ofadsorbers 3 in adsorber housing body 4. Each adsorber has a first end 5and a second end 6, with a flow path therebetween contacting anitrogen-selective adsorbent (for oxygen enrichment). The adsorbers aredeployed in an axisymmetric array about axis 7 of the adsorber housingbody. The housing body 4 is in relative rotary motion about axis 7 withfirst and second functional bodies 8 and 9, being engaged across a firstvalve face 10 with the first functional body 8 to which feed gas mixtureis supplied and from which the heavy product is withdrawn, and across asecond valve face 11 with the second functional body 9 from which thelight product is withdrawn.

[0055] In embodiments as particularly depicted in FIGS. 1-5, theadsorber housing 4 rotates and shall henceforth be referred to as theadsorber rotor 4, while the first and second functional bodies arestationary and together constitute a stator assembly 12 of the module.The first functional body shall henceforth be referred to as the firstvalve stator 8, and the second functional body shall henceforth bereferred to as the second valve stator 9. According to alternativeembodiments, the adsorber housing 4 may be stationary, while the firstand second functional bodies are rotary distributor valves.

[0056] In the embodiment shown in FIGS. 1-5, the flow path through theadsorbers is parallel to axis 7, so that the flow direction is axial,while the first and second valve faces are shown as flat annular discsnormal to axis 7. However, more generally the flow direction in theadsorbers may be axial or radial, and the first and second valve facesmay be any figure of revolution centred on axis 7. The steps of theprocess and the functional compartments to be defined will be in thesame angular relationship regardless of a radial or axial flow directionin the adsorbers.

[0057] FIGS. 2-5 are cross sections of module 1 in the planes defined byarrows 12′-13′, 14′-15′, and 16′-17′. Arrow 20 in each section shows thedirection of rotation of the rotor 4.

[0058]FIG. 2 shows section 12′-13′ across FIG. 1, which crosses theadsorber rotor. In this example, “N”=72. The adsorbers 3 are mountedbetween outer wall 21 and inner wall 22 of adsorber wheel 208. Eachadsorber comprises a rectangular flat pack 3 of adsorbent sheets 23,with spacers 24 between the sheets to define flow channels here in theaxial direction. Separators 25 are provided between the adsorbers tofill void space and prevent leakage between the adsorbers. In otherconfigurations, the adsorbent sheets may be formed in curved packs orspiral rolls.

[0059] Satisfactory adsorbent sheets have been made by coating a slurryof zeolite crystals with binder constituents onto the reinforcementmaterial, with successful examples including non-woven fibreglassscrims, woven metal fabrics, and expanded aluminium foils. The adsorbentsheets comprise a reinforcement material, in preferred embodiments glassfibre, metal foil or wire mesh, to which the adsorbent material isattached with a suitable binder. For applications such as hydrogenpurification, some or all of the adsorbent material may be provided ascarbon fibers, in woven or nonwoven form to serve as its ownreinforcement material. Spacers are provided by printing or embossingthe adsorbent sheet with a raised pattern, or by placing a fabricatedspacer between adjacent pairs of adsorbent sheets. Alternativesatisfactory spacers have been provided as woven metal screens,non-woven fibreglass scrims, and metal foils with etched flow channelsin a photolithographic pattern. The active adsorbent may be supported onthin adsorbent sheets which are layered and spaced apart by spacersdefining flow channels, so as to provide a high surface area parallelpassage support with minimal mass transfer resistance and flow channelpressure drop. With crystalline adsorbents such as zeolites, andamorphous adsorbents such as alumina gel or silica gel, the adsorbentsheet may be formed by coating or in-situ synthesis of the adsorbent ona reinforcement sheet of inert material, e.g. a wire mesh, a metal foil,a glass or mineral fiber paper, or a woven or non-woven fabric.Adsorbers made from the layered adsorbent sheet material may be formedby stacking flat or curved sheets; or by forming a spiral roll, with theflow channels between the sheets extending from the first end of theadsorber to the second end thereof; to fill the volume of the adsorberhousing of the desired shape. Examples of method and structures withpacked, spirally wound adsorbents are disclosed in commonly-owned,co-pending U.S. Provisional Application No. 60/285,527, filed Apr. 20,2001, and incorporated herein by reference.

[0060] Typical thickness of the adsorbent sheet may be in the range ofabout 100 to about 200 microns, while flow channel spacing between thesheets may be in the range of about 50 to about 200 microns. Typicalexperimental sheet thicknesses have been 150 microns, with spacerheights in the range of 100 to 150 microns, and adsorber flow channellength approximately 20 cm. Using X type zeolites, excellent performancehas been achieved in oxygen separation from air and hydrogenpurification from reformate at PSA cycle frequencies in the range of 1to at least 150 cycles per minute, particularly at least 25 cycles perminute.

[0061] The adsorbent material contacting the flow channels between thefirst and second ends of the adsorbers may generally be selected to bedifferent in distinct zones of the flow channels, so that the adsorberswould have a succession of zones (e.g. a first zone, a second zone, athird zone, a perhaps additional zones) with distinct adsorbentsproceeding along the flow channels from the first end to the second end.In a typical embodiment, the adsorbent in a first zone of the adsorbersadjacent the first end will be a desiccant to achieve bulk removal ofwater vapor in that first zone, the adsorbent in a second zone in thecentral portion of the adsorbers will be selected to achieve bulkremoval of nitrogen and any contaminant gas components, and theadsorbent in a third zone of the adsorbers will be selected to achievefurther removal of nitrogen. A suitable desiccant for the first zone isalumina gel. Suitable adsorbents for the second zone include 13X, or 5A,or Ca-X, or Ca-LSX zeolites. Suitable adsorbents for the third zoneinclude strongly nitrogen selective adsorbents selected from the groupincluding but not limited to Ca-LSX, Li-LSX, Li-exchanged chabazite,Ca-exchanged chabazite, and Sr-exchanged chabazite. The zeoliteadsorbents of this group are characterized by strong hydrophilicity,corresponding to selectivity for polar molecules. The second and thirdzones may be consolidated as a single zone using a single adsorbentcomposition. High performance conventional adsorbents will operate mosteffectively at relatively lower temperatures such as about 50-60° C. forLi-LSX or about 60-80° C. for Ca-LSX. Best performance for Na-X (13X) orNa-LSX may be achieved at much lower temperatures around approximately0° C., indicating a benefit for refrigeration of the feed air with suchadsorbents. Certain advanced zeolite adsorbents such as Ca- orSr-exchanged chabazite may be advantageously effective for nitrogenremoval at temperatures of about 100° C.

[0062] With specific reference to FIG. 1, the adsorbers 3 comprise aplurality of distinct zones between the first end 5 and the second end 6of the flow channels, here shown as three zones—a first zone 26 adjacentthe first end 5, a second zone 27 in the middle of the adsorbers, and athird zone 28 adjacent the second end 6. The first zone typicallycontains an adsorbent or desiccant selected for removing very stronglyadsorbed components of the feed gas mixture, such as water or methanolvapour, and some carbon dioxide. The second zone contains an adsorbenttypically selected for bulk separation of impurities at relatively highconcentration, and the third zone contains an adsorbent typicallyselected for polishing removal of impurities at relatively lowconcentration. Particularly in the first zone of the adsorber, theadsorbent must be compatible with significant concentrations of watervapor.

[0063] In embodiments with three zones, the first zone may be the first10% to 20% of the flow channel length from the first end, the secondzone may be the next roughly 40% to 50% of the channel length, and thethird zone the remainder. In embodiments with only two adsorber zones,the first zone may be the first 10% to 30% of the flow channel lengthfrom the first end, and the second zone the remainder. The zones may beformed by coating the different adsorbents onto the adsorbent supportsheet material in bands of the same width as the flow channel length ofthe corresponding zone. The adsorbent material composition may changeabruptly at the zone boundary, or may blend smoothly across theboundary. As an alternative to distinct zones of adsorbents, thedifferent adsorbents may be provided in layers or mixtures that includevarying gradients of adsorbent concentrations along the gas flow path. Afurther option is to provide a mixture of the different adsorbents thatmay or may not be homogenous.

[0064] For air separation to produce enriched oxygen, alumina gel may beused in the first zone to remove water vapour, while typical adsorbentsin the second and third zones are X, A or chabazite type zeolites,typically exchanged with lithium, calcium, strontium, magnesium and/orother cations, and with optimised silicon/aluminium ratios as well knownin the art. The zeolite crystals are bound with silica, clay and otherbinders, or self-bound, within the adsorbent sheet matrix.

[0065]FIG. 3 shows the porting of rotor 4 in the first and second valvefaces respectively in the planes defined by arrows 14′-15′, and 16′-17′.An adsorber port 30 provides fluid communication directly from the firstor second end of each adsorber to respectively the first or second valveface.

[0066]FIGS. 4A and 4B show the first stator valve face 100 of the firststator 8 in the first valve face 10, in the plane defined by arrows14′-15′. Fluid connections are shown to a feed compressor 101 inductingfeed gas from inlet filter 102, and to an exhauster 103 deliveringsecond product to a second product delivery conduit 104. Compressor 101and exhauster 103 are shown coupled to a drive motor 105.

[0067] Arrow 20 indicates the direction of rotation by the adsorberrotor. In the annular valve face between circumferential seals 106 and107, the open area of first stator valve face 100 ported to the feed andexhaust compartments is indicated by clear angular segments 111-116corresponding to the first functional ports communicating directly tofunctional compartments identified by the same reference numerals111-116. The substantially closed area of valve face 100 betweenfunctional compartments is indicated by hatched sectors 118 and 119 thatare slippers with zero clearance, or preferably a narrow clearance toreduce friction and wear without excessive leakage. Typical closedsector 118 provides a transition for an adsorber, between being open tocompartment 114 and open to compartment 115. A gradual opening isprovided by a tapering clearance channel between the slipper and thesealing face, so as to achieve gentle pressure equalization of anadsorber being opened to a new compartment. Much wider closed sectors(e.g. 119) are provided to substantially close flow to or from one endof the adsorbers when pressurization or blowdown is being performed fromthe other end.

[0068] The feed compressor provides feed gas to feed pressurizationcompartments 111 and 112, and to feed production compartment 113.Compartments 111 and 112 have successively increasing working pressures,while compartment 113 is at the highest working pressure of the PSAcycle. Compressor 101 may thus be a multistage or split streamcompressor system delivering the appropriate volume of feed flow to eachcompartment so as to achieve the pressurization of adsorbers through theintermediate pressure levels of compartments 111 and 112, and then thefinal pressurization and production through compartment 113. A splitstream compressor system may be provided in series as a multistagecompressor with interstage delivery ports; or as a plurality ofcompressors or compression cylinders in parallel, each delivering feedair to the working pressure of a compartment 111 to 113. Alternatively,compressor 101 may deliver all the feed gas to the higher pressure, withthrottling of some of that gas to supply feed pressurizationcompartments 111 and 112 at their respective intermediate pressures.

[0069] Similarly, exhauster 103 exhausts heavy product gas fromcountercurrent blowdown compartments 114 and 115 at the successivelydecreasing working pressures of those compartments, and finally fromexhaust compartment 116 which is at the lowest pressure of the cycle.Similarly to compressor 101, exhauster 103 may be provided as amultistage or split stream machine, with stages in series or in parallelto accept each flow at the appropriate intermediate pressure descendingto the lowest pressure.

[0070] In the example embodiment of FIG. 4A, the lowest pressure isambient pressure, so exhaust compartment 116 communicates directly toheavy product delivery conduit 104. Exhauster 103 thus is an expanderthat provides pressure letdown with energy recovery to assist motor 105from the countercurrrent blowdown compartments 114 and 115. Forsimplicity, exhauster 103 may be replaced by throttling orifices ascountercurrent blowdown pressure letdown means from compartments 114 and115.

[0071] In some embodiments, the lowest pressure of the PSA cycle issubatmospheric. Exhauster 103 is then provided as a vacuum pump, asshown in FIG. 4B. Again, the vacuum pump may be multistage or splitstream, with separate stages in series or in parallel, to acceptcountercurrent blowdown streams exiting their compartments at workingpressures greater than the lowest pressure which is the deepest vacuumpressure. In FIG. 4B, the early countercurrent blowdown stream fromcompartment 114 is released at ambient pressure directly to heavyproduct delivery conduit 104. If for simplicity a single stage vacuumpump were used, the countercurrent blowdown stream from compartment 115would be throttled down to the lower pressure over an orifice to jointhe stream from compartment 116 at the inlet of the vacuum pump.

[0072] If the feed gas is provided at an elevated pressure at leastequal to the highest pressure of the PSA cycle, compressor 101 would beeliminated. To reduce energy losses from irreversible throttling overorifices to supply feed pressurization compartments, the number of feedpressurization stages may be reduced, so that adsorber repressurizationis largely achieved by product pressurization, by backfill from lightreflux steps. Alternatively, compressor 101 may be replaced in part byan expander which expands feed gas to a feed pressurization compartmentfrom the feed supply pressure of the highest pressure to theintermediate pressure of that compartment, so as to recover energy fordriving a vacuum pump 103 which reduces the lowest pressure belowambient pressure so as to enhance the PSA process performance.

[0073]FIGS. 5A and 5B shows the second stator valve face, at section16′-17′ of FIG. 1. Open ports of the valve face are second valvefunction ports communicating directly to a light product deliverycompartment 121; a number of light reflux exit compartments 122, 123,124 and 125; and the same number of light reflux return compartments126, 127, 128 and 129 within the second stator. The second valvefunction ports are in the annular ring defined by circumferential seals131 and 132. Each pair of light reflux exit and return compartmentsprovides a stage of light reflux pressure letdown, respectively for thePSA process functions of supply to backfill, full or partial pressureequalization, and cocurrent blowdown to purge.

[0074] Illustrating the option of light reflux pressure letdown withenergy recovery, a split stream light reflux expander 140 is shown inFIGS. 1 and 5A to provide pressure let-down of four light reflux stageswith energy recovery. The light reflux expander provides pressurelet-down for each of four light reflux stages, respectively betweenlight reflux exit and return compartments 122 and 129, 123 and 128, 124and 127, and 125 and 126 as illustrated. The light reflux expander 140may power a light product booster compressor 145 by drive shaft 146,which delivers the oxygen enriched light product to oxygen deliveryconduit 147 and compressed to a delivery pressure above the highestpressure of the PSA cycle.

[0075] Light reflux expander 140 is coupled to a light product pressurebooster compressor 145 by drive shaft 146. Compressor 145 receives thelight product from compartment 121, and delivers light product(compressed to a delivery pressure above the highest pressure of the PSAcycle) to delivery conduit 147. Since the light reflux and light producthave approximately the same purity, expander 140 and light productcompressor 145 may be hermetically enclosed in a single housing whichmay conveniently be integrated with the second stator as shown inFIG. 1. This configuration of a “turbo-compressor” light product boosterwithout a separate drive motor is advantageous, as a useful pressureboost of the light product can be achieved without an external motor andcorresponding shaft seals, and can also be very compact when designed tooperate at very high shaft speeds.

[0076]FIG. 5B shows the simpler alternative of using a throttle orifice150 as the pressure letdown means for each of the light reflux stages.

[0077] Turning back to FIG. 1, compressed feed gas is supplied tocompartment 113 as indicated by arrow 125, while heavy product isexhausted from compartment 117 as indicated by arrow 126. The rotor issupported by bearing 160 with shaft seal 161 on rotor drive shaft 162 inthe first stator 8, which is integrally assembled with the first andsecond valve stators. The adsorber rotor is driven by motor 163 as rotordrive means.

[0078] A buffer seal 170 is provided to provide more positive sealing ofa buffer chamber 171 between seals 131 and 171. The buffer seal 170substantially prevents leakage across outer circumferential seal 131 onthe second valve face 11 that may compromise light product purity, andmore importantly may allow ingress of humidity into the second ends ofthe adsorbers which could deactivate the nitrogen-selective orCO-selective adsorbent. Even though the working pressure in some zonesof the second valve face may be subatmospheric (in the case that avacuum pump is used as exhauster 103), buffer chamber 171 is filled withdry light product gas at a buffer pressure positively above ambientpressure. Hence, minor leakage of light product outward may take place,but humid feed gas may not leak into the buffer chamber. In order tofurther minimize leakage and to reduce seal frictional torque, bufferseal 171 seals on a sealing face 172 at a much smaller diameter than thediameter of circumferential seal 131. Buffer seal 170 seals between arotor extension 175 of adsorber rotor 4 and the sealing face 172 on thesecond valve stator 9, with rotor extension 175 enveloping the rearportion of second valve stator 9 to form buffer chamber 171. A statorhousing member 180 is provided as structural connection between firstvalve stator 8 and second valve stator 9. Direct porting of adsorbers tothe stator face is an alternative to providing such seals and isdescribed in commonly-owned, co-pending U.S. Provisional Application No.60/301,723, filed Jun. 28, 2001, and incorporated herein by reference.

[0079] In the following FIGS. 6-13, simplified diagrams will represent aPSA apparatus or module. These highly simplified diagrams will indicatejust a single feed conduit 181 to, and a single heavy product conduit182 from, the first valve face 10; and the light product deliveryconduit 147 and a single representative light reflux stage 184 withpressure let-down means communicating to the second valve face 11. Thefuel cells referred to in FIGS. 6-13 are polymer electrolyte membrane(PEM) fuel cells, but the systems disclosed are useful with any type offuel cell such as, for example, a solid oxide fuel cell, or a moltencarbonate fuel cell. The disclosed systems and process are particularlyuseful for PEM fuel cell systems for which oxygen enrichment isespecially desirable. However, in certain fuel cell systems oxygenenrichment may not be present since air may be introduced directly intothe fuel cell. Thus, the oxygen enrichment PSA modules depicted in therepresentative systems shown in FIGS. 6-13 are optional.

[0080]FIG. 6

[0081]FIGS. 6A and 6B show a representative fuel cell power plant 200that includes a fuel cell 202, a hydrogen storage system 204, and anoxygen enrichment PSA or VPSA system 206. According to a particularrepresentative embodiment, the fuel cell comprises an anode channel 208including an anode gas inlet 210 and optionally an anode gas outlet 212,a cathode channel 214 including a cathode gas inlet 216 and a cathodegas outlet 218, and a PEM electrolyte membrane 220 in communication withthe anode channel 208 and the cathode channel 214 for facilitating ionexchange between the anode channel 208 and the cathode channel 214.

[0082] The oxygen enrichment system 206 extracts oxygen gas from feedair, and comprises a PSA rotary module 1 and a compressor 101 fordelivering pressurized feed air to the feed compartments of the PSArotary module 1. If configured as a VPSA system, the oxygen enrichmentsystem 206 would include a vacuum pump 103 (as shown in FIG. 8), whichmay be coupled to the compressor 101, for withdrawing nitrogen-enrichedgas as heavy product gas from the blowdown and exhaust compartments ofthe PSA rotary module 1, and discharging the nitrogen enriched gas fromconduit 225. Dry oxygen-enriched air as the light product gas of PSAmodule 1 is delivered by conduit 147 to humidification chamber 230, andthence by conduit 231 to cathode inlet 216. A portion of the oxygenreacts with hydrogen ions to form water in the cathode, and theremaining cathode gas containing this product water is withdrawn fromcathode exit 218. In order to achieve satisfactory fuel cell watermanagement, a portion of the product water may be optionally mixed backinto the cathode gas, most conveniently by recirculating all or part ofthe cathode gas from exit 218 by conduit 232 back to humidificationchamber 230. A boost pump 235 may be provided in conduit 231 to drivethe recirculation flow of the cathode exhaust gas. Excess water andcathode exhaust gas may be removed from conduit 232 by a water separator236 with cooler 237 and a discharge conduit 238 with control valve 239.According to particular embodiments, the humidification chamber 230 maybe integrated with the separator 236, with cathode purge removed byconduit 238 directly from conduit 232. Water condensate and excesscathode gas to be purged may be removed by separate discharge conduits.

[0083] Hydrogen storage system 204 includes a hydrogen storage vessel250. Hydrogen storage vessel 250 may be a simple pressure vessel forcompressed hydrogen, operating to pressures as high as 700 barsabsolute. Alternatively, hydrogen storage vessel 250 may be a pressurevessel container for a bed of a hydrogen sorbent. Illustrative hydrogensorbents include (for the embodiment of FIG. 6B) hydride-forming metalor metal alloys such as FeTi, LaNi₅ or Mg₂Ni; and physical adsorbentsincluding zeolites or carbon adsorbents such as activated carbon, carbonpowder, amorphous carbon, carbon fibers, carbon nanofibers, carbonnanotubes, and similar graphite materials as described, for example, inPCT Application Publication No. WO 00/75559. Further illustrativehydrogen storage systems include sodium borohydride and calciumborohydride that release compressed hydrogen upon contact with water.According to a further variant, hydrogen storage vessel 250 may be adewar with an insulation jacket 251 for containing liquid hydrogen at atemperature of about 20 K. Alternatively, hydrogen may be adsorbed in acryogenic adsorbent bed, such as a bed of activated carbon or a zeoliteadsorbent, at a temperature in the approximate range of 77K to ambient.

[0084] Hydrogen fuel is released from hydrogen storage vessel 250 byvalve 254 or a flow metering device as fuel flow control means (and thusas the fuel release mechanism) into conduit 256, and thence to ahydrogen energy-recovery expander 260. Hydrogen expander 260 ismechanically coupled to compressor 101 in FIG. 6A or to vacuum pump 103in FIG. 6B via schematically depicted shaft 261. Thus, hydrogen expander260 assists in powering compressor 101. If the compressor 101 is atwo-stage compressor, one stage might be powered by shaft 261 and theother stage by motor 105. Although not shown in FIG. 6, hydrogenexpander 260 could be coupled to other devices requiring mechanicalpower such as, for example, a vacuum pump, rotary bed or rotary valvefor the PSA module 1 or to external power loads such as vehiclepropulsion or air conditioning.

[0085] In general, the hydrogen may be released by the fuel flow controlmeans at an elevated pressure (e.g., about 700 to about 10 barsabsolute) relative to the fuel cell hydrogen working pressure (e.g.,about 1 to about 10 bars absolute). The hydrogen may also be released ata sub-ambient temperature corresponding to the storage temperature.Expander 260 lets down or reduces the pressure of the hydrogen in one ormore stages, to substantially the working pressure of the fuel cellanode inlet.

[0086] The expansion occurring in the expander 260 cools the hydrogen.In the embodiment depicted in FIG. 6A the cooled hydrogen may be warmedby heat exchange with feed air in a heat exchange coil 270 fluidlycommunicating with expander 260 by conduit 272 and with the anode inlet210 by conduit 274. Feed air to compressor 101 is introduced via inletfilter 280 and infeed conduit 281 to heat exchange condenser 282enclosing the heat exchange coil 270, and is countercurrently cooledagainst the hydrogen being warmed within the coil, before being suppliedto compressor 101 by conduit 283. Atmospheric humidity is partiallycondensed, and is removed from the condenser 282 by drain valve 285.Alternatively, the condensed atmospheric humidity could be introduced tothe humidification chamber 230. The cooled and dried air is thenconveyed by conduit 283 from condenser 282 to compressor 101. Thearrangement of the heat exchanger condenser 282 and the heat exchangecoil 270 is simply an illustrative example and alternative heat exchangearrangements could be employed. For example, it may be necessary toavoid frost build-up on coil 270 in winter conditions or it may bedesirable to allow the fuel cell system to heat up as quickly aspossible when the starting temperature is near or below 0° C. In thesecases, a conduit 286 with valve 287 bypassing the heat exchangercondenser 282 may be provided so that conduit 281 may be connecteddirectly to conduit 283 and the inlet of compressor 101 in cold weatherconditions. According to further variations of the embodiment shown inFIG. 6A, the hydrogen entering the heat exchange condenser 282 could beused to cool other fluid streams or components of the system in additionto, or instead of, the inlet air stream. For example, possible coolingopportunities exist for cathode recycle gas in conduit 232, other fuelcell reactants, or fuel cell components such as heat exchangersassociated with a fuel cell coolant loop.

[0087] A heat exchanger 290 is shown in communication between storagevessel 250 and condenser 282 to illustrate recovery of refrigerationfrom release of hydrogen stored under cryogenic conditions, or stored inan adsorbent such as activated carbon at sufficiently low temperaturesso that a useful cooling effect may be obtained from the endothermicheat of desorption. Heat exchanger 290 may represent conductive metalfins or rods extending from the interior of vessel 250 to heat exchangecondenser 282, or may represent heat exchange coils containing any heatexchange fluid circulated by free or forced convection, e.g., by anauxiliary pump. According to certain embodiments, heat exchanger 290 maybe a loop of heat exchange coil 270 extending into vessel 250, so thatalready at least partly warmed hydrogen fuel is used as the heatexchange fluid. Isolation valves (not shown) may be provided for heatexchanger 290 at its penetrations through vessel 250, with theseisolation valves opened when fuel is being delivered, and closed whenfuel is not being delivered so as to prevent heat exchange circulationwhen the power plant is shut down.

[0088]FIG. 6B shows a related embodiment, in which fuel cell waste heatis used to release the hydrogen fuel from storage vessel 250, and towarm the hydrogen in conduit 256 prior to its expansion acrossenergy-recovery expander 260. Expander 260 thus achieves energy recoveryfrom both the fuel cell waste heat and from the release of hydrogen fromthe fuel storage system. Cathode gas from cathode exit port 218 iscirculated by conduit 232 to the heat exchanger condenser 282, fromwhich excess water condensate is released by water discharge valve 239in conduit 238. Excess cathode gas (concentrated in nitrogen and argonafter consumption of oxygen in the fuel cell cathode) is purged by gasdischarge valve 239′ in conduit 238′. Heat is transferred from thecathode gas to the hydrogen in conduit 256 via heat exchanger 290.

[0089] The embodiment depicted in FIG. 6B recovers heat from cathodeexhaust gas being withdrawn (and in part recirculated) from the fuelcell cathode. The operating temperature of low temperature fuel cells(e.g. PEM fuel cells) is typically at least 60° C. and may be about 100°C. This is a convenient temperature for desorbing hydrogen from metalhydrides such as FeTi (which holds about 1.9% hydrogen by weight at ahydrogen pressure of about 5.2 bars at 30° C.) or LaNi₅ (which holdsabout 1.4% hydrogen by weight at a hydrogen pressure of about 4 bars at50° C.). By heating the metal hydride through heat exchanger 290 to atemperature approaching the working temperature of the fuel cell,hydrogen may readily be released into conduit 256 at an elevatedpressure in the range of about 4 to about 20 bars, thus providingdriving pressure for energy-recovery expander 260.

[0090] The operating temperature of high temperature fuel cells (e.g.SOFC fuel cells) is typically at least 600° C. and may be about 1000° C.Magnesium based hydrides have exceptionally high hydrogen capacity andare particularly suitable when high grade waste heat is available asfrom a SOFC fuel cell. For example, Mg₂Ni holds about 3.6% hydrogen byweight at a hydrogen pressure of about 2.5 bars at 300° C. By heatingsuch metal hydrides through heat exchanger means 290 to about 350° C., ausefully elevated inlet pressure in the range of about 10 to about 20bars may be provided to energy recovery expander 260.

[0091] While heat recovery from the fuel cell cathode loop has beendepicted in FIG. 6B, it will be appreciated that waste heat may berecovered to a heat exchange condenser 282 from an anode gasrecirculation loop or from any other heat source (such as an anode tailgas combustor) in the fuel cell system.

[0092] In the case of cryogenic storage of liquid hydrogen, arefrigeration effect may be obtained by vaporization and then by warmingup of the hydrogen at substantially ambient pressure. The cryogenicliquid hydrogen may be warmed, and thus vaporized, via heat transferfrom the heat exchanger 290 to the cooler cryogenic liquid hydrogen.This refrigeration effect is used in heat exchanger 290 to chill thefeed air in countercurrent heat exchange with the hydrogen being warmed.A greater refrigeration effect plus recovered mechanical energy may beobtained by pumping the liquid hydrogen to an elevated substantiallysupercritical pressure (e.g. 100 or 200 bars) and then warming up thehydrogen via heat exchanger 290 prior to several stages of expansion inexpander 260.

[0093] An even greater refrigeration effect plus recovered mechanicalenergy may be achieved by providing the heat to warm up the hydrogen asheat of compression rejected by a multistage Stirling engine (or anengine using a similar regenerative thermodynamic cycle to the Stirlingcycle). The expansion portion of the Stirling engine cycle also couldabsorb heat from the feed air, thus chilling the feed air prior to itsintroduction into an oxygen PSA system. The heat taken up by theStirling cycle to chill the feed air is much greater than the heatrejected at low temperature by the same Stirling cycle to warm thehydrogen. In these examples, the useful refrigeration effect (to chillfeed air to the PSA unit) is greatly enhanced by the recovery ofmechanical energy to help drive the PSA compressor and/or vacuum pump.An example of a system that includes a Stirling engine is describedbelow in more detail in connection with FIG. 11.

[0094] Chilling the feed air to the PSA unit also reduces power demandof the PSA feed air compressor by reducing the volume of air to becompressed, both by reducing the temperature of the feed air and byreducing the mole fraction of water vapor. Systems with multistage feedair compression (as detailed below) may employ intercooling between thestages for reducing frost formation without foregoing the benefit ofreducing the volume of feed air to the compressor. Power demand of thePSA unit may be further reduced by chilling to an optimum temperaturefor achieving high fractional yield of oxygen from the feed air, thusagain reducing the volume of feed to be compressed and the volume ofexhaust flow for vacuum pumping. For example, the temperature of thefeed air introduced into the oxygen enrichment system 206 may be reducedto a temperature of about 0 to about 10° C. at the compressor 101 inletat summer conditions. In winter conditions, this chilling step would bebypassed, so the PSA system would operate under approximately similarinlet temperature conditions during all seasons.

[0095] The embodiment depicted in FIG. 6 depicts a system for capturingadditional hydrogen storage energy from the cooling effect that occursin each stage of expansion in expander 260 when the hydrogen is cooledby approximately ideal isentropic expansion. However, such additionalenergy recovery may not be necessary in all systems. Thus, the hydrogenexiting the expander 260 may be introduced directly into the anode gasinlet 210 without first passing through the heat exchange coil 270.

[0096] Optionally, a PSA module (not shown) may be located between theexpander 260 and/or heat exchange condenser 282 for purifying orenriching the hydrogen gas entering the anode gas inlet 210.Illustrative hydrogen-enriching PSA modules are described, for example,in commonly-assigned, copending U.S. patent application filed Oct. 26,2001, for “Systems and Process for Providing Hydrogen to Fuel Cells.”

[0097]FIG. 7

[0098]FIG. 7 shows a temperature-entropy diagram 300 for one gram ofhydrogen under compressed and/or cryogenic storage conditions asprovided in Perry and Chilton, “Chemical Engineers' Handbook”(McGraw-Hill). The ordinate 301 is absolute temperature, and theabscissa 302 is entropy. The phase change between vapor and liquid isdenoted by phase boundary curve 303. Below the phase boundary curve, theone atmosphere isobar 310 for mixed liquid and vapor is indicated by asolid line extending from liquid 311 to vapor 312 on phase boundarycurve 303.

[0099] In the gas phase, the one atmosphere isobar 314 is shownextending from vapour point 312 to point 315 at a nominal ambienttemperature of 25° C. Similarly, the isobar for 2 bars pressure is shownas a dashed line 320 in the mixed phase zone and as a dashed curve 321for gas up to point 322 at 25° C. A supercritical isobar 340 for apressure of 100 bars is shown extending from point 341 (reached byisentropic compression of liquid from point 311) to point 342 at 25° C.A dashed line 350 indicates the typical 80° C. working temperature of aPEM fuel cell.

[0100] Point 342 (100 bars, 25° C.) indicates a typical starting pointfor energy recovery from hydrogen stored as compressed gas. Sincetypical compressed hydrogen refueling pressure would be much higher,e.g. at least 200 bars and with current composite pressure vessels up toabout 700 bars absolute at fully charged pressure, this starting pointactually corresponds to energy recovery from a hydrogen storage tankwhich is at least half discharged from its “full” condition. Expansionenergy recovery is achieved in this example by three stages ofexpansion, a first stage from point 342 to point 360, a second stagefrom point 362 to point 363, and a third stage from point 365 to point366—and with each stage of expansion over a similar pressure ratio sothat each stage recovers a similar amount of mechanical energy andprovides a similar temperature reduction to the other stages. In thisinstance, the hydrogen is re-heated for each expansion (e.g., from point360 to point 362, from point 363 to point 365, etc.) by coils 270 shownin FIG. 8. Assuming that expansion of the first stage starts from 100bars pressure, and expansion of the third stage concludes at 2 barspressure as the working pressure of the fuel cell the total pressureratio of the three-stage expansion is 50. Hence, the pressure ratio ofeach expansion stage may be the cube root of 50, or approximately 3.8.

[0101] With typical efficiency of the expansion stages, each stage ofexpansion will cool the hydrogen by approximately 65 K. That coolingeffect will be transferred in a heat exchanger coil 270 to refrigeratethe feed air as the hydrogen is reheated for the next stage of expansionor finally for the fuel cell. Assuming a 15 K approach in heat exchange,about 50 K of reheating the hydrogen will be available for cooling thefeed air. Low humidity air passing through condenser 282 should becooled by about 30 K since the molar flow of feed air will typically beabout 5 times the molar feed flow of the fuel hydrogen (assuming 60%fractional recovery of oxygen by the PSA from the feed air, 80%utilization of the enriched oxygen by the fuel cell, and 100%utilization of the hydrogen by the fuel cell) and the cooling effect isprovided three times. The temperature reduction would, of course, beless for high humidity air, owing to reduction of the sensible heatavailable for temperature reduction by the latent heat of water beingcondensed.

[0102] In the alternative case that hydrogen is stored as cryogenicliquid at about 20K, the starting point for “direct expansion cycle”energy recovery would be pumping the liquid hydrogen from point 311 (onebar pressure) to point 341 (e.g. 100 bars pressure). The hydrogen isthen warmed along supercritical isobar 340 from point 341 to point 342at approaching ambient temperature, from which energy recovery expansionmay take place in three stages (points 342-360, 362-363, 365-366)exactly as discussed above.

[0103]FIG. 7 also shows the opportunity for complementing the directexpansion cycle with further energy recovery by a regenerativethermodynamic cycle such as the Stirling cycle. A two stage Stirlingcycle will be described in FIG. 11 below. The idealized T-S diagram of atwo stage Stirling cycle includes heat rejection by the Stirling engineat a lower temperature (e.g., about 50 K to about 60 K) along the linebetween points 370 and 371, heat rejection by the Stirling engine at anintermediate temperature (e.g. between about 100 K and about 150 K)between points 372 and 373, and heat uptake or cooling by the Stirlingengine at an upper temperature (e.g., about 300 K to about 360 K) ofbetween points 376 and 377. The upper temperature may be seasonablyvariable and somewhat below ambient as when cooling feed air to the PSAunit and condensing humidity from that feed air. Alternatively, theupper temperature could be substantially the working temperature of thefuel cell stack, so that waste heat from the fuel cell may be recoveredeither directly by cooling the stack 202 or indirectly from heatexchanger 237 on the cathode exhaust gas. While PEM fuel cells operatein the approximate range of about 80° C. to about 100° C., highertemperature fuel cells (e.g. solid oxide fuel cells) operate at veryhigh temperatures in the range of about 600° C. to about 1000° C. sothat high grade stack waste heat is generated. The thermodynamicefficiency of energy recovery from fuel cell stack waste heat is greatlyenhanced when heat can be rejected at a cryogenic temperature asprovided in the presently described systems. In such embodiments energyrecovery from cryogenic hydrogen storage is synergistically combinedwith energy recovery from fuel cell stack waste heat.

[0104] The area enclosed by points 370, 371, 372, 373, 377, 376 and backto point 370 indicates the theoretical work which could be extracted(expressed as calories of equivalent work per gram of hydrogen releasedas fuel) by an ideal two stage Stirling cycle of perfect efficiency. Theenergy availability or exergy for ideally efficient release of hydrogen,to ambient pressure and temperature from liquid hydrogen storage, isgiven by the area on the T-S diagram enclosed by points 311, 312, 315,376 and back to point 311, above the one atmosphere isobar 314.

[0105] FIGS. 8-10

[0106] FIGS. 8-10 are schematics of a representative fuel cell powerplant 400 with compressed hydrogen storage in high pressure vessel 250.According to the examples shown in FIGS. 8-10, expander 260 is providedas three expander stages 401, 402 and 403, each delivering hydrogen gasthat has been cooled by approximately isentropic expansion into a heatexchange coil 270. However, expander 260 may include any suitable numberof stages such as, for example, two stages or four stages. Separate heatexchange coils may be provided to each expander stage or the streamsfrom each expander stage may be combined and introduced into a singleheat exchange coil. The hydrogen is warmed in the heat exchange coils270 by heat exchange with feed air in condenser 282, so that the air iscooled and a portion of the atmospheric humidity may be condensed out.

[0107] As the pressure in vessel 250 drops with progressive delivery ofits contained compressed fuel, the pressure across each expander stage401, 402, 403 will also drop. Consequently, the availability of energyrecovery from the fuel being delivered also declines. If desired, one ormore stages 401-403 may be bypassed when the pressure remaining invessel 250 is significantly reduced. Such by pass avoids an undesirablylow pressure ratio across the other expander stages which would thenperform the full expansion duty.

[0108] FIGS. 8-10 also show optional recirculation devices for thecathode and the anode loops of the fuel cell, provided, respectively, asejectors 411 and 412. These ejectors or other recirculation blowersassist in maintaining humidity balance in all parts of the cathode andanode flow channels.

[0109] Ejector 411 defines a nozzle 413 for receiving oxygen-enrichedair from the PSA module 1 via a conduit 147 and a non-return valve 412.Recirculated oxygen-enriched cathode gas is brought by conduit 232 andwater separator 236 to an ejector inlet 414. Ejector 414 inlet can be asuction inlet. The combined flow from the nozzle 413 and the suctioninlet 414 is mixed in mixing section 415 of the ejector, followed bypressure recovery in diffuser 416 before the flow is brought by conduit231 to the cathode inlet 216. Similarly, ejector 412 defines a nozzle423 for receiving hydrogen from the hydrogen storage vessel 250 via aconduit 274. Recirculated anode exhaust gas is brought from the anodechannel exit port 212 by conduit or anode gas loop 424 to an ejectorinlet 425. Ejector inlet 425 can be a suction inlet. The combined flowfrom the nozzle 423 and the suction inlet 425 is mixed in mixing section426 of the ejector, followed by pressure recovery in diffuser 427 beforethe flow is brought by conduit 428 to the anode inlet 210.

[0110] As shown in FIGS. 9A and 9B, a purge line 430 with purge valve431 may be provided to purge inert components from the anode gas loop424. Purge may be required during initial startup, and a small level ofpurge during operation will be required if the feed hydrogen is lessthan absolutely pure. For higher levels of impurity, a hydrogen PSA unitmay be located between the heat exchange condenser 282 and the anodeinlet 210 to remove impurity components while minimizing loss of fuelhydrogen. With a system operating on highly pure hydrogen fuel, anoderecirculation and anode purging may not be necessary.

[0111] In FIG. 8, the oxygen enrichment is performed by VPSA. Thethree-stage hydrogen storage energy recovery expander is shownschematically coupled by shaft 261 to assist a motor 105 in drivingcompressor 101 and vacuum pump 103. Of course, compressor 101 and pump103 could be driven by separate motors, and the energy recovery could beapplied to either one of them. For example, FIG. 10 depicts a systemwherein the compressor 101 and the vacuum pump 103 are de-coupled fromeach other. The compressor 101 is powered by motor 105. The vacuum pump103 is powered by the shaft 261 coupled to the expander 260.

[0112]FIGS. 9A and 9B include a PSA system in which the feed air iscompressed in two stages, the first stage being compressor 101 poweredby electric motor 105, and the second stage being compressor 450 whichis powered by expander 260 through shaft 261. Conduit 451 can beconfigured (not shown) to pass through heat exchange condenser 282 inorder to provide an intercooling function between the compressionstages. Second stage compressor 450 may be a centrifugal or multistageaxial compressor, and expander 260 may be an impulse turbine asdescribed above. Second stage compressor 405 and expander 260 togetheroperate as a free rotor “turbocharger” 460. In FIG. 9A (as in FIGS. 8and 10) feed air to the oxygen PSA unit 206 is cooled and dehumidifiedin heat exchange condenser 282. FIG. 9B shows the alternative of fuelcell waste heat from the cathode being recovered in heat exchangecondenser 282 similar to FIG. 6B.

[0113]FIGS. 11A and 11B

[0114]FIGS. 11A and 11B show another representative fuel cell powerplant 500 with liquid hydrogen stored in dewar 250 which has aninsulation jacket 251 and a delivery valve 254 in delivery conduit 256.

[0115] In a first aspect of expansion energy recovery by a modifieddirect expansion cycle, the liquid hydrogen is pumped to an elevatedpressure that in the illustrative example of FIG.7 is 100 bars. Theliquid hydrogen is pumped from point 311 to point 341 of FIG. 7 bycryogenic pump 510 that may be a piston pump. The piston pump mayinclude a reciprocating piston either mechanically driven by acrankshaft coupled to a motor or to shaft 261, or else electricallydriven by a reciprocating linear electric motor.

[0116] The hydrogen is then warmed along supercritical isobar 340 frompoint 341 to point 342 (see FIG. 7), in a first heat exchanger 520 and asecond heat exchanger 521 in conduit 522 communicating from pump 510 tohydrogen expander 260. The warmer heat transfer stream in closeproximity to the hydrogen in conduit 522 in first and second heatexchangers 520, 521 may be air from the system feed air inlet, a fuelcell exhaust gas stream, or a Stirling engine working fluid as describedin more detail below. First and second heat exchangers 520 and 521 couldbe used for further cooling of the feed air exiting from heat exchangecondenser 282 to achieve a further reduction in humidity and also in aircompression power, or alternatively could be used as sinks for fuel cellstack waste heat. For example, the first heat exchanger may warm thehydrogen to an intermediate temperature between 100 K and 150 K, whilethe second heat exchanger may warm the hydrogen to approximately ambienttemperature. Hydrogen storage energy recovery is then achieved inexpander 260 as discussed for FIGS. 8-10, in FIG. 11A with a usefulrefrigeration effect on feed air through heat exchange coil(s) 270 inheat exchange condenser 282. In FIG. 11B, fuel cell waste heat from thecathode is recovered in heat exchange condenser 282, similar to FIGS. 6Band 9B.

[0117] A desirable variant is to include a container or bed 523 ofortho-para hydrogen conversion catalyst between first and second heatexchangers 520 and 521. A suitable catalyst is iron impregnated activealumina or hydrous ferric oxide. Hydrogen stored in the parahydrogenspin isomer (the stable isomeric form for liquid hydrogen) upon contactwith the ortho-para conversion catalyst will partly convertendothermically to the orthohydrogen isomer. The orthohydrogen isomerconstitutes about 75 mole % fraction of hydrogen gas at equilibrium andambient temperature. By increasing the quantity and reversibility ofheat take-up at low temperature, this reverse ortho-para conversion willenable a further improvement in energy recovery as fuel hydrogen isreleased from liquid hydrogen storage.

[0118] Frost accumulation may occur in the first and/or second heatexchangers 520, 521 particularly due to the very low temperatures of thefirst heat exchanger, which would make defrosting relatively difficult.Another potential issue arises with the thermodynamic inefficiency ofusing heat at or near ambient temperature for warming a cryogenic fluidat much lower temperatures.

[0119] These frost accumulation and thermodynamic inefficiencies couldbe overcome if a regenerative thermodynamic engine cycle is used toaccept heat at an upper temperature near ambient temperature (e.g., heatfrom cooling feed air and condensing humidity, or alternatively fuelcell stack waste heat at somewhat higher than ambient temperature), andto reject heat at a lower temperature corresponding to the warming ofthe cryogenic hydrogen. The regenerative engine cycle then deliversmechanical power to recover a fraction of the hydrogen storage energypreviously expended in the fuel supply infrastructure to liquefyhydrogen. Because of the conversion of thermal to mechanical energy, theregenerative engine cycle accepts much more heat at the uppertemperature than it rejects at the lower temperature, thus leveragingits desirable cooling effect by the recovery of mechanical power hereapplied to compressing air into the oxygen PSA.

[0120] More specifically, FIGS. 11A and 11B illustrate a second aspectof energy recovery from liquid hydrogen storage. In particular, aStirling engine 530 is provided for recovering mechanical energy andobtaining an enhanced cooling effect from the release of fuel hydrogenfrom cryogenic storage. While FIGS. 11A and 11B illustrate thecombination of direct expansion and Stirling cycle energy recoverysystems, it is to be understood that either of these energy recoverysystems may be applied independently without the other. For example, inthe case of a Stirling cycle energy recovery system alone, the presenceof the expander 260 that performs at least a portion of the directexpansion cycle energy recovery is not required. A regenerative Braytoncycle engine may be provided as an alternative to the Stirling engine530 as an example of an engine based on a regenerative thermodynamiccycle.

[0121] The Stirling engine 530 of FIG. 11A is a two-stage machine havinga first regenerator 531 and a second regenerator 532 that has a largergas volume than first regenerator 531. Each regenerator 531, 532 definesa flow path between first and seconds ends thereof, with solid material(e.g., wire mesh packing) having heat storage capacity disposed alongthe flow path and in thermal contact with gas in the flow path. TheStirling engine 530 has a working volume including the first and secondregenerators 531, 532 and three cyclic displacement chambers, includinga first chamber 534, a second chamber 535 and a third chamber 536. Theworking volume is filled with any suitable working fluid such ashydrogen or alternatively helium. The working fluid and the fuel gas maybe substantially identical. Hydrogen is the preferred working fluid whenhydrogen is the fuel gas.

[0122] The first chamber 534 is in fluid communication with a first endof first regenerator 531 via conduit 580. Conduit 580 includes a heatexchange coil 541 located between the first chamber 534 and the firstregenerator 531. The heat exchange coil 541 is proximally disposed toheat exchanger 520 such that heat is exchanged from the warmer Stirlingengine working fluid to the cooler hydrogen fuel. The second chamber 535is in fluid communication with a first end of second regenerator 532 anda second end of first regenerator 531 via conduit 581. Conduit 581includes a heat exchange coil 542 located between the second chamber 535and the first and second regenerators 531, 532. The heat exchange coil542 is proximally disposed to heat exchanger 521 such that heat isexchanged from the warmer Stirling engine working fluid to the coolerhydrogen fuel. The third chamber 536 is in fluid communication with thesecond end of second regenerator 532 via conduit 582. Conduit 582includes a heat exchange coil 543 located between the third chamber 536and the second regenerator 532. Heat exchange coil 543 is disposedwithin condenser 282 such that heat is exchanged from the coolerStirling engine working fluid to the warmer feed air. The Stirlingengine 530 can run at a relatively high speed (e.g., about 1000 to about3000 RPM) so that the heat exchanges described above are smoothed out tobe effectively continuous.

[0123] Alternatively, heat exchange coil 543 could be in heat exchange(not shown) with the fuel cell stack exhaust stream or separator 236 forrecovery of fuel cell waste heat. In such an alternative embodiment, theheat exchange coil 543 and a conduit carrying a fuel cell cathodeexhaust stream and/or a fuel cell anode exhaust stream could both bedisposed within a heat exchanger such that heat is transferred from thefuel cell exhaust stream to the Stirling engine working fluid.

[0124] The mechanism of the Stirling engine 530 causes cyclic volumechanges to take place within the first, second and third chambers 534,535, 536, respectively, at the working frequency of the Stirling engine530. The relative phases of these cyclic volume changes arepredetermined to achieve cyclic variations of total working space volumeand the pressure within the working space. These cyclic variations arecoordinated with cyclic reversals of working fluid flow in the first andsecond regenerators 531, 532. The flow direction of the working fluidthrough the Stirling working volume is typically directed towards thehigher temperature end of each regenerator 531, 532 when the pressure inthat regenerator is higher than the mean working pressure within thedisplacement chambers 534, 535, and 536 of the Stirling engine 530. Theflow direction of the working fluid through the Stirling working volumeis typically directed towards the lower temperature end of eachregenerator 531, 532 when the pressure in that regenerator is lower thanthe mean working pressure within the displacement chambers 534, 535, and536 of the Stirling engine 530. The phase of cyclic volume changes inthe first and second chambers 534, 535 should lag the phase of volumechanges in the third chamber 536, typically by about 90°, for generatingheat for warming the hydrogen fuel in heat exchangers 520 and 521 whilesimultaneously removing heat from the feed air via heat exchange coil543.

[0125] In the particular illustrative Stirling engine configuration 530,coaxial cylinders 550 and 551 define the first and second chambers 534and 535, respectively. A stepped piston 555 is received within cylinders550,551. The cylinders 550,551 engage the stepped piston 555 such thatthe stepped piston 555 can undergo axial movement relative to thecylinders 550,551. An additional cylinder 556 defines the third chamber536. An additional piston 557 is received within the cylinder 556. Thecylinder 556 engages the piston 557 such the piston 557 can undergoaxial movement relative to the cylinder 556. Pistons 555 and 557 arerespectively coupled by connecting rods 560 and 561 to crank pin 562 oncrank 563, which revolves on crankshaft 564 within crankcase 565. Thedirection of rotation of crankshaft 564 is clockwise as shown by arrow566. Coaxial cylinders 550 and 551 are mounted with a right angledoffset to cylinder 556 in order to establish the desired phase relationof cyclic displacements in the first, second and third chambers 534,535, 536. Dashed line 570 represents a mechanical coupling fromcrankshaft 564 to assist driving compressor 101, or alternatively avacuum pump 103 in a VPSA configuration. Of course, alternative Stirlingengine designs could be employed such as a single cylinder,piston-plus-displacer design.

[0126] In the energy recovery operating mode as described above, theStirling engine 530 delivers mechanical power while rejecting heat towarm the cryogenic fuel. The Stirling engine 530 may be operated inreverse either by reversing the rotational direction (opposite thenormal direction shown by arrow 566) of the crankshaft 564 or byreversing the phase so that cyclic volume displacements in the first andsecond chambers 534, 535 have a leading phase with respect to cyclicvolume changes in the third chamber 536. Thus operating in reverse, theStirling engine 530 consumes power to operate as a cryogenicrefrigerator. When the fuel cell power plant is shut down for extendedtime-periods, evaporation of stored liquid hydrogen becomes a majorproblem. Reverse operation of the Stirling engine 530 may then beperformed at intervals to condense the hydrogen vapor boil-off orsub-cool the stored liquid hydrogen to reduce the vapor pressure andprevent evaporation. Using an external power source to operate theStirling engine 530 in its reverse refrigeration mode may be initiatedautomatically when the pressure within the liquid hydrogen storage tankexceeds a specified pressure setting, above which evaporation would takeplace.

[0127] While Stirling engine 530 is depicted as having a crankshaft 564for mechanically coupling its pistons to each other and to its load(e.g. a vacuum pump or a compressor) by a shaft 570, an alternativeapproach provides that a Stirling engine piston be directly coupled to areciprocating cylinder of a compressor 101 or a vacuum pump 103. Thecrankshaft may then be used to synchronize piston timing, withoutdelivering power to the external load. In free piston embodiments ofStirling engines, the crankshaft coupling may be eliminated.

[0128]FIG. 11B illustrates a three stage Stirling engine 530 in whichthe stepped piston 555 and its associated coaxial cylinders 550, 551 areprovided with an additional step defining an intermediate chamber 535′cooperating through heat exchange coil 542′ to an intermediateregenerator 531′ disposed in the flow path between regenerators 531 and532. Heat exchange coil 542′ is coupled to heat exchanger 521′ in whichthe fuel hydrogen is warmed at a temperature level intermediate betweenthe temperature level in heat exchangers 521 and 270. A refuelingconnector 572 is fluidly coupled to hydrogen storage vessel 250 forre-supplying the hydrogen storage vessel 250. A fuel inlet 570 isfluidly coupled to the hydrogen storage vessel for introducing fuel intofuel delivery conduit 571.

[0129]FIG. 12

[0130]FIG. 12 shows a temperature-entropy diagram for hydrogen, with athree stage Stirling cycle for energy recovery from hydrogen fuelrelease from liquid hydrogen storage, but with no direct expansion cycleenergy recovery. A three stage Stirling engine may be provided byincluding a third regenerator and an extra chamber relative to theStirling engine 530 as illustrated in FIG. 11B. For example, the steppedpiston 555 and its associated coaxial cylinders 550, 551 could define anadditional step. Corresponding to the extra chamber, the three stageStirling cycle would reject heat to a second intermediate temperaturelevel defined by points 581 and 582.

[0131]FIG. 13

[0132]FIG. 13 shows a fuel cell power plant 600 that includes a systemfor defrosting the heat exchange coil 270 in the condenser 282 so as toprevent ice build-up. A second heat exchange coil 270′ co-operating witha second condenser 282′ is provided in parallel with heat exchange coil270 co-operating with condenser 282. Heat exchange coils 270 and 270′are in parallel between conduits 272 and 274, and respectively havehydrogen shutoff valves 601 and 602 so that hydrogen flow through oneheat exchange coil may be stopped while that coil is being defrosted.Feed air shutoff valves 611 and 612 are provided to connect condensers282 and 282′, respectively, to conduit 283 and compressor 101. Airexhaust shutoff valves 621 and 622 are provided to connect condensers282 and 282′, respectively, to conduit 225 and thence either to thedischarge of vacuum pump 103 or directly to the exhaust port of the PSAmodule. Condenser drain valves 285 and 285′ for condensers 282 and 282′,respectively, are shown as non-return valves. An air exhaust vent valve630 is provided for exhaust discharge from conduit 225 whenever bothexhaust shutoff valves 621 and 622 are closed.

[0133] When one of coils 270 or 270′ is being defrosted, its respectivehydrogen shutoff valve 601 or 602 is closed, its respective feed airshutoff valve 611 or 612 is closed, and its respective air exhaustshutoff valve 621 or 622 is opened. During non-defrosting operation,coils 270 or 270′ cool incoming feed air and condense out humidity andtheir respective hydrogen shutoff valve 601 or 602 is open, feed airshutoff valve 611 or 612 is open, and air exhaust shutoff valve 621 or622 is closed. Since the defrost time interval will be relatively short,a plurality of more than two heat exchange coils and condensercombinations (each with its associated hydrogen shutoff valve, feedshutoff valve, and exhaust shutoff valve) may be provided in parallel sothat less than half of the feed cooling capacity is shut down during anydefrosting interval.

[0134]FIGS. 14 and 15

[0135]FIGS. 14 and 15 illustrate a representative rotary valve pressureswing adsorption apparatus that could be used in the presently describedsystems and processes. In particular, FIG. 14 depicts a stationary bedsystem, where the feed ends of adsorbers 803 use a rotary valve tosynchronize flows. The light product end uses some valve switching inorder to affect a PSA process. Feed gas is transported via conduit 813to heavies valve 867, through dynamic seal 860 and rotor body 861,rotating about axis 862 by motor 863. Feed flow is directed to seal 864and through stator housing 865 to adsorber 803. Exhaust gases aredirected from adsorber 803 through stator housing 865, seal 864, androtor body 861. The fluids are contained by second stator housing 866 incoordination with stator housing 865, and withdrawn via conduit 817.

[0136] The light product end of the adsorbers 803 are depicted asconventional conduit with directional valves 868 used to providesynchronized pressure and flow cycling in coordination with the feed endvalve 867, and the adsorbers 803, with the product fluid being deliveredby product conduit 847. Note that this drawing depicts only the simplest2-adsorber PSA and that it represents all PSA configurations with arotary feed valve and conventional valve arrangements for the lightproduct end fluids. The light product end system is completely enclosedin an impermeable container 870, where tight fluid sealing is achievedacross the whole boundary. In this option, atmospheric bornecontaminants are not able to enter into the process across the valvestem actuators, which are the process containment seals. The staticbuffer space (the space around the valves bounded by static sealing) ispreferably filled with a buffer fluid, introduced by a buffer fluidsupply leading to port 871. A positive pressure gradient over theambient pressure is a preferred option. This buffer fluid is alsopreferably circulated and refreshed by allowing the fluid to bewithdrawn by port 872.

[0137] One way valve 869 can be used to minimize reverse flow of anycontaminant coming from down stream equipment or processes, as well asthe preferred option of using product gas as the buffer fluid by closingvalve 874 and allowing the product fluid to enter container 870 viavalve 873, and to allow the product to be withdrawn from the container873 through product conduit 875.

[0138]FIG. 15 also depicts a rotary PSA system, wherein the lightproduct end of adsorbers 803 uses a multi-port rotary distributor valveto synchronize pressure and flow cycles. The lights valve 878 contains arotor 879 being rotated by motor 880, and where dynamic seals 881communicate with the adsorbers 803 in a cyclic manner. Feed gas isallowed in conduit 813 to a set of directional valves 876, and is thendirected to one of the adsorbers 803, where product gas is drawn offthrough seal 881, through lights rotor 879, and into product conduit 847via dynamic seal 882 and product port 883. The dynamic seals 881 and 882are process containment seals, and in the configuration where lightsvalve housing 884 is not sealed, they are also the primary seal, andhave the least amount of resistance to contaminant ingress from thesurrounding atmosphere. In one option, the housing 884 can be sealed, inorder to create a static buffer space that can be protected as discussedabove. Another option is to allow the static buffer chamber to breathethrough breather 885 coupled to blanket gas supply 886. Anotherpreferred option is to allow the static buffer chamber to breathethrough breather 887, and preferably through guard trap 888. Exhaustgases are withdrawn from adsorber 803 via directional valve 876 andthrough conduit 817.

[0139] A combination of devices shown in FIGS. 14 and 15, such asheavies valve 867, coupled to adsorbers 803 and to lights valve 878 isalso considered a rotary PSA. A system consisting of the light productend valves 868 with associated conduits, along with adsorbers 803 andfirst end valves 878 and associated conduits consist of conventionalPSA.

[0140]FIG. 16

[0141]FIG. 16 shows a fuel cell power plant 800 with a regenerativeBrayton cycle engine 910 for combined energy recovery from the fuel cellstack waste heat and from a cryogenic hydrogen storage system. Theworking fluid of the Brayton cycle engine is illustrated as hydrogen.The cryogenic fuel being warmed by the engine passes directly throughthe engine cycle in this example, although the fuel hydrogen could bewarmed indirectly by the engine working fluid as in the above examplesbased on Stirling engines.

[0142] Engine 910 includes an expander 911, a first engine compressor912 and a second engine compressor 913 which are mechanically coupled byshaft 915. The engine works between an upper pressure and a lowerpressure of the Brayton cycle; and in this embodiment expander 911,first engine compressor 912 and second engine compressor 913 all workbetween substantially the same upper and lower pressures (apart fromflow friction pressure drops in conduits and heat exchangers) whileoperating at different temperatures. Engine 910 is coupled by shaft 261to a mechanical load, here illustrated as vacuum pump 103 and compressor101 which may also be powered in part by motor 105. Compressor 101provides feed air to an oxygen enrichment PSA unit whose exhaust gas isextracted by vacuum pump 103. Motor 105 may serve as a starter motor forengine 910. In the case that the fuel cell is a high temperature typesuch as a solid oxide fuel cell, the power output of engine 910 mayexceed the power demand of compressor 101 and vacuum pump 103, so motor105 may then function as an electrical generator to deliver excess powerrecovered by the Brayton cycle engine from fuel cell stack heat andcryogenic hydrogen storage.

[0143] Fuel cell power plant 800 has liquid hydrogen stored in dewar 250which has an insulation jacket 251 and a liquid delivery valve 254 indelivery conduit 256. Delivery conduit 256 delivers liquid hydrogen tovaporizer 920 that is warmed by heat exchange coil 924. Hydrogen gas isdelivered from vaporizer 920 by conduit 925 to join Brayton engine lowerpressure conduit 930 feeding hydrogen to the inlet of first enginecompressor 912. Compressor 912 delivers compressed hydrogen atsubstantially the upper pressure of the Brayton cycle to Brayton engineupper pressure conduit 940 which delivers the hydrogen to heat exchangecoil 924 (thus providing heat of compression from the first enginecompressor to the vaporizer 920).

[0144] Upper pressure conduit 940 extends through heat exchange coil 924in the vaporizer, a first recuperator stage 942, a second recuperatorstage 944, and heater coil 270 as the hydrogen is warmed fromsubstantially its cryogenic storage temperature to an upper temperatureof the Brayton cycle approaching the exit temperature of the fuel cellstack. Hydrogen working fluid having been heated by coil 270 is thenexpanded by the engine expander 911 to the lower pressure of the Braytoncycle, and is delivered by conduit 950 communicating for fuel deliveryto the fuel cell anode inlet port 216 and to the Brayton engine lowerpressure conduit 930 returning hydrogen engine working fluid backthrough recuperator stages 944 and 942 to the inlet of first enginecompressor 912. The working fluid mass flow is larger in recuperatorstage 944 than in first recuperator stage 942.

[0145] The second engine compressor 913 works between the upper andlower pressures in an intermediate temperature range between ambienttemperature and the vaporizer temperature, compressing gas from thelower pressure conduit 930 to the upper pressure conduit 940 betweenrecuperators 942 and 944. It delivers heat of compression at anintermediate cryogenic temperature to the hydrogen flowing in the upperpressure conduit toward the warmer end of the engine. Heat ofcompression from the first engine compressor 912 thus releases thehydrogen fuel by vaporization from its cryogenic storage phase(corresponding to the heat transfer between points 370 and 371 in FIG.7), while heat of compression from the second first engine compressor913 assists in warming the fuel (corresponding to the heat transferbetween points 372 and 373 in FIG. 7). Heat rejection (as heat ofcompression) at lower temperatures by the regenerative Brayton engineallows it to recovery more fuel cell waste heat (as heat of expansion),while a wide temperature span between the upper and lower temperaturesof the regenerative Brayton cycle will enhance its thermal efficiency.While this embodiment of the invention may be applied to any fuel celltype, highest efficiency is achieved by any regenerative engine cyclewhen high grade heat can be provided to the engine from a hightemperature fuel cell system or from a high temperature component of thefuel cell system. The ability to reject heat from a regenerative enginecycle to the very low temperatures of a liquid hydrogen storage systemwill greatly enhance atttainable efficiency of the engine cycle.

[0146] If desired to further improve efficiency of the regenerativeBrayton cycle engine, one or more additional recuperator stages could beprovided, together with an engine compressor between each adjacent pairof recuperator stages spanning sub-ambient temperatures where heat mayusefully be provided from the engine cycle to warm the hydrogen fuelgas. In other variants, fuel cell waste heat may be recovered to heatexchanger coil 270 from the cathode gas stream as depicted, oralternatively from the anode gas stream or from elsewhere in the fuelcell system. It will also be noted that fuel gas from vaporizer 920could be supplied to the upper pressure conduit rather than the lowerpressure conduit, so as to recover some liquefaction energy by directexpansion. Also, the hydrogen fuel gas could be provided to the fuelcell anode at the upper pressure rather than the lower pressure of theregenerative Brayton cycle. It may also be noted that rotaryregenerators could be substituted for stationary recuperative heatexchangers of the recuperator stages.

We claim:
 1. A power plant system that can use a fuel that is a gas atambient temperature and pressure, comprising: at least one power plant;at least one fuel storage container; and at least one expander that canreceive fuel from the fuel storage container at a first pressure andprovide the fuel to the power plant at a second pressure that is lowerthan the first pressure.
 2. The system according to claim 1, wherein thepower plant comprises a fuel cell.
 3. The system according to claim 1,wherein the power plant comprises a combustion engine.
 4. The systemaccording to claim 1, wherein the fuel storage container is selectedfrom a pressure vessel for holding compressed gas, a pressure vessel fora bed of a gas sorbent, and a dewar for containing a liquefied gas. 5.The system according to claim 2, wherein the fuel storage containerholds compressed hydrogen gas or cryogenic liquid hydrogen.
 6. Thesystem according to claim 2, wherein the expander is coupled to at leastone device selected from a compressor, a pump, an adsorber rotor, or avehicle propulsion device.
 7. The system according to claim 2, whereinthe fuel storage container holds cryogenic liquid hydrogen, the powerplant system further comprising at least one heat exchanger containing aworking fluid, the heat exchanger being juxtaposed with the fuel storagecontainer such that heat can be transferred from the working fluid tothe fuel in the fuel storage container.
 8. The system according to claim2, further comprising a first conduit fluidly communicating between theexpander and the fuel cell for carrying the fuel, wherein at least aportion of the first conduit is disposed within at least one heatexchanger such that the fuel is a coolant.
 9. The system according toclaim 1, wherein the fuel comprises hydrogen, methane, natural gas, orpropane.
 10. A power plant system that can use a fuel that is a gas atambient temperature pressure, comprising: at least one power plant; atleast one fuel storage container; a first conduit fluidly coupling thefuel storage container and the power plant for delivering fuel from thefuel storage container to the power plant; and at least one regenerativethermodynamic cycle engine thermally coupled to the first conduit suchthat heat may be exchanged between the fuel and a working fluid for theregenerative thermodynamic cycle engine.
 11. The system according toclaim 10, wherein the power plant comprises a fuel cell.
 12. The systemaccording to claim 10, wherein the power plant comprises a combustionengine.
 13. The system according to claim 10, wherein the fuel storagecontainer is selected from a pressure vessel for holding compressed gas,a pressure vessel for a bed of a gas sorbent, and a dewar for containinga liquefied gas.
 14. The system according to claim 11, wherein the fuelstorage container holds compressed hydrogen gas or cryogenic liquidhydrogen.
 15. The system according to claim 11, wherein the regenerativethermodynamic cycle engine is coupled to at least one device selectedfrom a compressor, a pump, an adsorber rotor, or a vehicle propulsiondevice.
 16. The system according to claim 15, wherein the regenerativethermodynamic cycle engine comprises a Stirling engine.
 17. The systemaccording to claim 16, further comprising at least one expander fluidlycoupled to the first conduit between the fuel storage container and thefuel cell such that the expander can receive fuel from the fuel storagecontainer at a first pressure and provide the fuel to the fuel cell at asecond pressure that is lower than the first pressure.
 18. The systemaccording to claim 17, further comprising: at least one first heatexchanger fluidly coupled to the first conduit between the expander andthe fuel cell; at least one pressure swing adsorption module defining aninlet that is in fluid communication with a second conduit for carryingan air feed stream; wherein at least a first portion of the firstconduit and at least a portion of the second conduit are disposed withinthe first heat exchanger such that heat can be transferred from the airfeed stream to the fuel.
 19. The system according to claim 18, furthercomprising a third conduit for carrying the working fluid of theStirling engine, at least a first portion of the third conduit beingdisposed within the first heat exchanger such that heat can betransferred from the air feed stream to the working fluid of theStirling engine.
 20. The system according to claim 19, furthercomprising a second heat exchanger housing at least a second portion ofthe first conduit and at least a second portion of the third conduitsuch that heat can be transferred from the working fluid of the Stirlingengine to the fuel.
 21. The system according to claim 16, furthercomprising: a second conduit for carrying an exhaust gas stream from thefuel cell; a third conduit for carrying the working fluid of theStirling engine; and a heat exchanger housing at least a portion of thesecond conduit and at least a portion of the third conduit such thatheat may be transferred from the exhaust gas stream to the working fluidof the Stirling engine.
 22. The system according to claim 21, whereinthe fuel comprises hydrogen, methane, natural gas, or propane.
 23. Anelectrical current generating system, comprising: at least one fuelcell; a fuel storage system; and means for converting energy fromrelease of fuel from the fuel storage system into mechanical power, heattransfer, or mechanical power and heat transfer.
 24. The systemaccording to claim 23, wherein the means for converting energy comprisesat least one device selected from an expander, a heat exchanger or aregenerative thermodynamic cycle engine.
 25. The system according toclaim 24, further comprising at least one gas delivery system that candeliver a gas to the fuel cell, the gas delivery system including atleast one mechanically-powered apparatus mechanically coupled to atleast one of the expander or regenerative thermodynamic cycle engine.26. The system according to claim 24, further comprising at least onegas delivery system that can deliver a gas to the fuel cell via aconduit that is thermally coupled to at least one heat exchanger suchthat heat can be exchanged between the gas and the fuel.
 27. Anelectrical current generating system, comprising: at least one fuelcell; at least one hydrogen storage system; at least one expander thatcan receive hydrogen from the hydrogen storage system at a firstpressure and provide the hydrogen to the fuel cell at a second pressurethat is lower than the first pressure; and at least one oxidant gasdelivery system that can produce oxidant-enriched gas for delivery tothe fuel cell and that includes at least one device that is coupled tothe expander.
 28. The system according to claim 27, wherein the oxidantgas delivery system comprises an oxygen gas delivery system thatincludes a pressure swing adsorption module and the device coupled tothe expander is selected from a compressor, vacuum pump, rotaryadsorbent bed and rotary adsorber valve.
 29. The system according toclaim 28, further comprising: a first conduit for carrying an air feedstream to the pressure swing adsorption module; a second conduit forcarrying the hydrogen from the expander to the fuel cell; and a heatexchanger housing at least a portion of the first conduit and at least aportion of the second conduit such that heat can be transferred from theair feed stream to the hydrogen.
 30. The system according to claim 27,wherein the hydrogen storage system comprises at least one containerselected from a pressure vessel for holding compressed hydrogen gas, apressure vessel for a bed of a hydrogen sorbent, and a dewar forcontaining liquid hydrogen.
 31. The system according to claim 28,wherein the pressure swing adsorption module comprises a rotary pressureswing adsorption module.
 32. The system according to claim 28, whereinthe hydrogen storage system holds cryogenic liquid hydrogen, theelectrical current generating system further comprising: a first conduitfor carrying an air feed stream to the pressure swing adsorption module;and at least one heat exchanger juxtaposed with the hydrogen storagesystem such that heat can be transferred from the air feed stream to thecryogenic liquid hydrogen.
 33. The system according to claim 27, whereinthe expander comprises a multistage expander.
 34. The system accordingto claim 27, wherein the expander comprises a positive displacementexpander or an impulse turbine.
 35. An electrical current generatingsystem, comprising: at least one fuel cell; at least one hydrogenstorage system; at least one oxidant gas delivery system that canproduce oxidant-enriched gas for delivery to the fuel cell; a firstconduit for carrying an air feed stream to the oxidant gas deliverysystem; and at least one Stirling engine thermally coupled to the firstconduit such that heat may be exchanged between the air feed stream anda working fluid for the Stirling engine.
 36. The system according toclaim 35, wherein the oxidant gas delivery system comprises an oxygengas delivery system that includes a pressure swing adsorption systemhaving at least one device coupled to the Stirling engine.
 37. Thesystem according to claim 36, wherein the device coupled to the Stirlingengine is selected from a compressor, vacuum pump, rotary adsorbent bedand rotary adsorber valve.
 38. The system according to claim 35, whereinthe hydrogen storage system comprises at least one container selectedfrom a pressure vessel for holding compressed hydrogen gas, a pressurevessel for a bed of a hydrogen sorbent, and a dewar for containingliquid hydrogen.
 39. The system according to claim 36, wherein thepressure swing adsorption module comprises a rotary pressure swingadsorption module.
 40. The system according to claim 35, furthercomprising at least one expander that can receive hydrogen from thehydrogen storage system at a first pressure and provide the hydrogen tothe fuel cell at a second pressure that is lower than the firstpressure.
 41. The system according to claim 40, wherein the expandercomprises a multistage expander.
 42. The system according to claim 40,wherein the expander comprises a positive displacement expander or animpulse turbine.
 43. The system according to claim 35, furthercomprising: a second conduit for carrying hydrogen from the hydrogenstorage system to the fuel cell; a third conduit for carrying theworking fluid of the Stirling engine; and a heat exchanger housing atleast a portion of the second conduit and at least a portion of thethird conduit such that heat may be transferred from the hydrogen to theworking fluid of the Stirling engine.
 44. The system according to claim40, further comprising: a second conduit for carrying hydrogen from thehydrogen storage system to the expander; a third conduit for carryingthe working fluid of the Stirling engine; a fourth conduit for carryinghydrogen from the expander to the fuel cell; a first heat exchangerhousing at least a portion of the second conduit and at least a firstportion of the third conduit such that heat may be transferred from thehydrogen to the working fluid of the Stirling engine; and a second heatexchanger housing at least a portion of the first conduit, at least asecond portion of the third conduit and at least a portion of the fourthconduit such that heat may be transferred from the air feed stream tothe hydrogen and the working fluid of the Stirling engine.
 45. Thesystem according to claim 35, further comprising: a second conduit forcarrying hydrogen from the hydrogen storage system to the expander; andan orthohydrogen-parahydrogen catalyst bed fluidly coupled to the secondconduit.
 46. The system according to claim 27, further comprising: afirst conduit for carrying an air feed stream to the oxidant gasdelivery system; a second conduit for carrying the air feed stream tothe oxidant gas delivery system; a third conduit for carrying hydrogenfrom the hydrogen storage system to the fuel cell; a fourth conduit forcarrying hydrogen from the hydrogen storage system to the fuel cell; afirst heat exchanger housing at least a portion of the first conduit andat least a portion of the third conduit for transferring heat from theair feed stream to the hydrogen; a second heat exchanger housing atleast a portion of the second conduit and at least a portion of thefourth conduit for transferring heat from the air feed stream to thehydrogen; a first feed air shutoff valve fluidly coupled to the firstconduit between the first heat exchanger and the oxidant gas deliverysystem; a second feed air shutoff valve fluidly coupled to the secondconduit between the second heat exchanger and the oxidant gas deliverysystem; a first feed air exhaust valve fluidly coupled to the firstconduit between the first heat exchanger and the oxidant gas deliverysystem; a second feed air exhaust valve fluidly coupled to the secondconduit between the second heat exchanger and the oxidant gas deliverysystem; a first hydrogen shutoff valve fluidly coupled to the thirdconduit between the hydrogen storage system and the first heatexchanger; and a second hydrogen shutoff valve fluidly coupled to thefourth conduit between the hydrogen storage system and the second heatexchanger.
 47. A process for providing fuel to a power plant,comprising: providing a fuel selected from compressed fuel gas and acryogenic liquid fuel; releasing the fuel from a fuel storage system;and generating mechanical power, a refrigeration effect, or mechanicalpower and a refrigeration effect from the releasing of the fuel.
 48. Theprocess according to claim 47, wherein the generation of the mechanicalpower, refrigeration effect, or mechanical power and refrigerationeffect from the releasing of the fuel comprises: releasing the fuel fromthe fuel storage system to provide a compressed fuel gas stream; andmechanically expanding the compressed fuel gas stream undersubstantially isentropic conditions.
 49. The process according to claim48, further comprising converting energy from the expansion of thecompressed fuel gas stream into mechanical power for driving at leastone device selected from a compressor, a pump, an adsorber rotor, and avehicle propulsion device.
 50. The process according to claim 48,wherein the expansion of the compressed fuel gas stream cools the fuelgas stream, the process further comprising: providing an air feedstream; transferring heat from the air feed stream to the cooled fuelgas stream resulting in cooling the air feed stream; introducing thecooled air feed stream into a pressure swing adsorption module toproduce an oxygen-enriched gas stream; and introducing theoxygen-enriched gas stream into the fuel cell.
 51. The process accordingto claim 47, wherein the fuel comprises cryogenic liquid fuel, theprocess further comprising: providing an air feed stream; transferringheat from the air feed stream to the cryogenic liquid fuel such that theair feed stream is cooled and the cryogenic liquid fuel is vaporizedinto a fuel gas; introducing the cooled air feed stream into a pressureswing adsorption module to produce an oxygen-enriched gas stream;introducing the oxygen-enriched gas stream into the fuel cell; andintroducing the fuel gas into the fuel cell.
 52. The process accordingto claim 47, wherein the compressed fuel gas has a pressure of greaterthan about 100 bars absolute.
 53. The process according to claim 47,wherein the fuel comprises hydrogen, methane, natural gas, or propane.54. A process for providing hydrogen to at least one fuel cell,comprising: releasing hydrogen from a hydrogen fuel storage system toprovide a compressed hydrogen gas stream; introducing the compressedhydrogen gas stream into at least one expander resulting in alower-pressure hydrogen gas stream; and introducing the lower-pressurehydrogen gas stream into a fuel cell.
 55. The process according to claim54, further comprising: providing an air feed stream; transferring heatfrom the air feed stream to the lower-pressure hydrogen gas stream suchthat the air feed stream is cooled; introducing the cooled air feedstream into a pressure swing adsorption system to produce anoxygen-enriched gas stream; and introducing the oxygen-enriched gasstream into the fuel cell.
 56. The process according to claim 54,wherein the hydrogen in the hydrogen fuel storage system comprisescryogenic liquid hydrogen, the process further comprising: providing anair feed stream; transferring heat from the air feed stream to thecryogenic liquid hydrogen stream such that the air feed stream is cooledand the cryogenic liquid hydrogen stream is vaporized into thecompressed hydrogen gas stream; introducing the cooled air feed streaminto a pressure swing adsorption system to produce an oxygen-enrichedgas stream; and introducing the oxygen-enriched gas stream into the fuelcell.
 57. The process according to claim 56, further comprising pumpingthe cryogenic liquid hydrogen to a substantially supercritical pressure.58. The process according to claim 54, further comprising driving atleast one device selected from a compressor, a pump, an adsorber rotor,and a vehicle propulsion device via a mechanical coupling between thedevice and the expander.
 59. The process according to claim 55, whereinthe pressure swing adsorption system includes at least one deviceselected from a pump and a compressor, the process further comprisingdriving the device via a shaft mechanically coupling the device with theexpander.
 60. The process according to claim 56, wherein the pressureswing adsorption system includes at least one device selected from avacuum pump and a compressor, the process further comprising driving thedevice via a shaft mechanically coupling the device with the expander.61. A process for providing hydrogen to at least one fuel cell,comprising: releasing hydrogen from a hydrogen fuel storage system toprovide a hydrogen stream; providing an air feed stream; providing aregenerative thermodynamic cycle engine having a working fluid;transferring heat from the regenerative thermodynamic cycle engineworking fluid to the hydrogen stream; transferring heat to theregenerative thermodynamic cycle engine working fluid from at least oneof the air feed stream and a fuel cell exhaust gas stream; introducingthe hydrogen stream into the fuel cell; and introducing the air feedstream into the fuel cell.
 62. The process according to claim 61,wherein the regenerative thermodynamic cycle engine comprises a Stirlingengine.
 63. The process according to claim 62, wherein the hydrogen inthe hydrogen fuel storage system comprises cryogenic liquid hydrogen,the process further comprising: pumping the cryogenic liquid hydrogen toa substantially supercritical pressure; transferring heat from theStirling engine working fluid to the cryogenic liquid hydrogen atsubstantially supercritical pressure resulting in a compressed hydrogengas stream; and introducing the compressed hydrogen gas stream into anexpander prior to introducing the hydrogen stream into the fuel cell.64. The process according to claim 62, wherein the hydrogen in thehydrogen fuel storage system comprises compressed hydrogen gas, theprocess further comprising: introducing the compressed hydrogen gasstream into an expander after the transferring of heat from the Stirlingengine working fluid to the compressed hydrogen gas stream resulting ina cooled hydrogen gas stream; and transferring heat from the air feedstream to the cooled hydrogen gas stream.
 65. The process according toclaim 62, wherein the transferring of heat from the air feed stream tothe Stirling engine working fluid results in a cooled air feed stream,the process further comprising: introducing the cooled air feed streaminto a pressure swing adsorption system to produce an oxygen-enrichedgas stream; and introducing the oxygen-enriched gas stream into the fuelcell.
 66. The process according to claim 65, wherein the pressure swingadsorption system includes at least one device selected from a pump anda compressor and the hydrogen in the hydrogen fuel storage systemcomprises compressed hydrogen gas, the process further comprising:introducing the compressed hydrogen gas stream into an expander afterthe transferring of heat from the Stirling engine working fluid to thecompressed hydrogen gas stream; and driving the pressure swingadsorption system device via a shaft mechanically coupling the pressureswing adsorption device with the expander.
 67. The process according toclaim 65, wherein the pressure swing adsorption system includes at leastone device selected from a pump and a compressor, the process furthercomprising: driving the pressure swing adsorption system device via ashaft mechanically coupling the pressure swing adsorption device withthe Stirling engine.
 68. The process according to claim 62, the processfurther comprising contacting the hydrogen stream with anorthohydrogen-parahydrogen catalyst.
 69. The process according to claim64, wherein the transferring of heat from the air feed stream to thecooled hydrogen gas stream results in a cooled air feed stream, theprocess further comprising: introducing the cooled air feed stream intoa pressure swing adsorption system to produce an oxygen-enriched gasstream; and introducing the oxygen-enriched gas stream into the fuelcell.
 70. The system according to claim 4, wherein the fuel storagecontainer comprises a pressure vessel that includes a bed of a physicaladsorbent.
 71. The system according to claim 70, wherein the adsorbentis selected from a carbon material and a zeolite.
 72. The systemaccording to claim 4, wherein the fuel comprises hydrogen and the fuelstorage container comprises a pressure vessel that includes a bed ofhydride forming metal or metallic alloy.
 73. The system according toclaim 10, wherein the working fluid for the regenerative thermodynamiccycle engine is substantially identical to the fuel gas.
 74. The systemaccording to claim 10, wherein the working fluid for the regenerativethermodynamic cycle engine and the fuel gas comprise hydrogen.
 75. Thesystem according to claim 13, wherein the fuel storage containercomprises a pressure vessel that includes a bed of a physical adsorbent.76. The system according to claim 75, wherein the adsorbent is selectedfrom a carbon material and a zeolite.
 77. The e system according toclaim 13, wherein the fuel comprises hydrogen and the fuel storagecontainer comprises a pressure vessel that includes a bed of hydrideforming metal or metallic alloy.
 78. The system according to claim 2,further comprising: a first conduit for carrying an exhaust gas streamfrom the fuel cell; and at least one heat exchanger juxtaposed with thefuel storage container such that heat can be transferred from theexhaust gas stream to fuel in the fuel storage container.
 79. Theprocess according to claim 61, further comprising intermittentlytransferring heat from the hydrogen stream to the regenerativethermodynamic cycle engine working fluid.
 80. The system according toclaim 29, further comprising a third conduit bypassing the heatexchanger for carrying the air feed stream to the pressure swingadsorption module.